if i set a workflow with some rule criteria that ..it will fire after 10 days. so if i change the rule criteria again .then is it fire on same day as its sheduled previouslly ??
Yes , if you change the rule criteria , it will not put any impact on time triggers unless and until you explicitly change the scheduled date. so it will fire on same day as it scheduled previously .
Time-dependent actions remain in the workflow queue only as long as the workflow rule criteria are still valid. If a record no longer matches the rule criteria, Salesforce removes the time-dependent actions queued for that record.
Have a look at this link : https://help.salesforce.com/HTViewHelpDoc?id=workflow_time_action_considerations.htm&language=en_US
for e.g: if I say the rule criteria is 10 days after when case closure - the Wworkflow will fire 10 days after the case is closed.
so it will fire on same day as it scheduled previously .
Have a look at this link : https://help.salesforce.com/HTViewHelpDoc?id=workflow_time_action_considerations.htm&language=en_US