Rule of thumb I've heard when overseeding is to water three times a day
for about 15 minutes each watering. Do this for about two weeks and
then return to your normal watering schedule. My guess is two weeks of
the altered schedule won't affect the existing lawn too much since it
is only for a short period of time.

I'm by no means an expert as we've just completed our first overseeding
last weekend. I'm just reciting the advice I was given.

Best of luck.