How long have you suspected she was pregnant?
Just that she looks ready to pop and if we're talking only a few fry then
3 weeks is about average I think. Could be another week if she's expecting more - don't have a clue how to tell though!
BTW - platties are known to miscarry if they don't have the right conditions ie the fry die and the mother re-absorbs them. Sounds yucky I know, but it happens.
If she's looking ready to pop, you might want to isolate her in a fry net or use your quarantine tank. That is, if you actually want the fry!
Ami