My mistake, I didn't look at the parameter for the setToAddresses.  Since it takes more than one e-mail address, and you're sending the exact same message to everybody, then you could accumulate the e-mail addresses in a List and just use that in ONE Messaging.SingleEmailMessage call.  OR, you could just make it an array of size=1 and use multiple SingleEmailMessages....  There is a limit of 100 e-mail addresses in one call, so it sort of depends on how many people you are going to be sending messages to.  

So, it's either this:  which uses one e-mai message, but has 100 addresses on the To: line

 public void doMail() {
        // Load the Profiles we want to compare against.
        Profile ConsultantProfile = [select id from profile where name='Consultant User'];
        Profile ProjectManagerProfile = [select id from profile where name='Project Manager'];

        // A place to hold the e-mail addresses we're accumulating
        List<String> emailAddresses = new List<String>();

        // A place to hold the Users we're going to check
        List<User> users = new List<User>([SELECT id,ProfileId,FirstName,Email,Total_hours_of_week__c FROM User WHERE User.Total_hours_of_week__c <: 40.00 limit 1000]);
         
        // A place to hold the Users we're going to update.  (Can't just use 'users' because we won't update all of them, only the ones with the right profiles)
        List<User> usersToUpdate = new List<User>();

        // Loop through the users we want to send messages to.
        for (User u: users) {
             system.debug('u is :: ' + u);

             // If the user isn't a Consultant or Project Manager, Skip them.
             if (u.ProfileId != ProjectManagerProfile.id && u.ProfileId != ConsultantProfile.id) continue;

             emailAddresses.add(u.Email);

             // Reset their hours
             u.Total_hours_of_week__c = 0.0;
             usersToUpdate.add(u);
         }
          
         if (emailAddress.isEmpty()) return;  // Nothing to do.

         // Build a message for the email.
         Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
         mail.setToAddresses(emailAddresses);
         mail.setsubject('Less Than 40 Hours for Last Week.');
         mail.setHtmlBody('You have less than 40 hours entered in Salesforce.com for last week.  Please update your hours or enter PTO.  If you have questions or are unable to enter your hours, please contact your System Administrator.');

         Messaging.sendEmail(new Messaging.Email[] {mail});   // Send the one message, but the To: list would have lots of e-mail addresses.
         // Note that setToAddresses is limited to 100 addresses.
         // Note that you might want to use CC or BCC, but those are limited to 25 addresses.
         
         update usersToUpdate;
        }
}

or this which just changes that one line.  This might be the better approach.  

global class NotificationScheduledFor40Hours implements Schedulable {
   
  global void execute(SchedulableContext ctx) {
      doMail();
  }
 
  public void doMail() {
        // Load the Profiles we want to compare against.
        Profile ConsultantProfile = [select id from profile where name='Consultant User'];
        Profile ProjectManagerProfile = [select id from profile where name='Project Manager'];

        // A place to hold the messages we're going to send
        List<Messaging.SingleEmailMessage> emails = new List<Messaging.SingleEmailMessage>();

        // A place to hold the Users we're going to check
        List<User> users = new List<User>([SELECT id,ProfileId,FirstName,Email,Total_hours_of_week__c FROM User WHERE User.Total_hours_of_week__c <: 40.00 limit 1000]);
         
        // A place to hold the Users we're going to update.  (Can't just use 'users' because we won't update all of them, only the ones with the right profiles)
        List<User> usersToUpdate = new List<User>();

        // Loop through the users we want to send messages to.
        for (User u: users) {
             system.debug('u is :: ' + u);

             // If the user isn't a Consultant or Project Manager, Skip them.
             if (u.ProfileId != ProjectManagerProfile.id && u.ProfileId != ConsultantProfile.id) continue;

             // Reset their hours
             u.Total_hours_of_week__c = 0.0;
             usersToUpdate.add(u);

             // Build a message for that user.
             Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();

// Change this:
//             mail.setToAddresses(u.Email);
// to this:
             mail.setToAddresses(new String[]{u.Email});


             mail.setsubject('Less Than 40 Hours for Last Week.');
             mail.setHtmlBody('You have less than 40 hours entered in Salesforce.com for last week.  Please update your hours or enter PTO.  If you have questions or are unable to enter your hours, please contact your System Administrator.');
            
             // Add it to the list of emails to send.
             emails.add(mail);
        }

        // If we have messages to send, send them and update the users.
        if (!emails.isEmpty()) {
             Messaging.sendEmail(emails);
             update usersToUpdate;
        }
}

Best, Steve;