After the re-buy period has ended and players have purchased add-ons, if available, no more chips can be bought and the tournament continues exactly like a regular freezeout tournament.
Time and money considerations
There are two important differences you need to be aware of when entering a re-buy tournament. Firstly and obviously, it requires a greater investment, on average. Sure, with some luck you can double up early and play the whole tournament on just one buy-in. But you shouldn’t count on it. Refusing to re-buy will decrease your chances compared to those of players who re-buy more liberally.
Secondly, a re-buy tournament lasts longer than a freezeout tournament with the same number of entrants. First of all there is the obvious extension caused by the re-buy period at the start of the tournament. On top of that, however, after a lot of re-buys have been made, the freezeout part of the tournament will often start at much deeper average chip stacks than a regular freezeout tournament, which makes it last longer.
Consider this when planning your poker session, or you may find yourself pretty exhausted next day at work!