I dont think you mean strobe warning, I think you mean entry time (time allowed to walk to the panel and type in your code when you open the front door, etc).
If so, this has to be set per zone, and if the power was lost then the programming has probably been lost too.
Below is a link to the engineering manual (assuming it is a post-2000 panel), though if you are not confident with setting this up then I recommend contacting an alarm installer. They shouldn't charge too much to help. To be honest, there should be a battery in your alarm that should have stopped the alarm going off in the first place, so it probably does need a service.
http://www.moretonalarms.com/pdf_engineer_manuals/Veritas8_INS034-12.pdf
I hope this helps.
Chris
Chris, June 2007