I am on flecainide 100mg twice a day. i am not sure about the interval between doses. does it have to be exactly 12 hours?

Not exact: An hour or two variation won't make a difference .

Flecainide dosing: Approximately every 10-12 hours is optimal. Don't extend beyond 12 hours between doses, and don't shorten to less than 9 hours between doses. This medication is important, and you don't want your serum levels to drop below therapeutic level or be potentially too high.