Ok heres a few of mine that i topped a while ago now theyve also been through a little lst... anyway like zandor said top at 4th or 5th node, that explains itself..
Q1 It isnt the exact stage but you can see in one of the pics where i have cut it..

Q2 A plant will take time to heal and shoot out 2 new nodes this can take anywhere from a few days to a week or more depending on how much light your plants are getting.. i would wait a few weeks after topping to flower to let some of the undergrowth from lower in the plant catch up with the top..

Q3 It depends on the strain some can double in bud mass others will stay the same..

Anyway hope i helped with your little problem... good luck btw sorry for the crappy pics they realy dont do my plants any justice at all...!!