[Mark's Sussex Allotment]

When people say things are pot bound.

What exactly does it mean?

Does it mean the plant has some sort of internal mechanism to stop growing, even if potted up in to a bigger pot?

Is there a certain timeframe before the plant decides that it's had enough, and is no longer viable for a sucessful transplantation?

[mumofstig]

It just means that the plant roots have filled the pot. Most plants will respond to potting on, bigger plants may need to have a few of the roots teased out to free them, as sometimes they can continue going round in circles - rather than growing into the fresh compost.
Most veg don't stay in their pots long enough to need that but shrubs and fruit trees often do.