If you have a condition which you know or suspect may render you unfit for flight you have to ground yourself.
You would then contact your AME or the CAA to see what needs to happen before you can be certified fit again.
Of course one can elect not to do this but you would be flying with an invalid medical and therefore you would hand your insurance an easy get out clause if anything happened. Not very sensible.
So, no there is no grace period.
FD