How high can I set undo retention on Oracle 11gR2

Question

Is it viable to set a very high undo retention, as to allow flashback queries several weeks back in time?

Naturally, enough space must be available in the undo tablespaces to contain the amount of undo data required. Are there other limitations I should be aware of?

What happens if there is not enough undo space available? Will production be affected in any way, or is it just a matter of flashback and rollback being limited (snapshot too old etc).

Update:

With a typical undo generation of a little less than 1 GB per day per instance, and up to 64 GB worth of undo space per instance it sounds viable to run with a 30-day undo retention target. No?