I am having an umbilical hernia repaired tomorrow morning, so i'll let you know how it goes! like anything else it is entirely dependant to the individual.
i first had the same repair done almost 20 years ago, and at the age of 17 and in tremendous shape i was out of action for only 3 weeks before being given the green light to start training again at full intensity.
at 37 years of age and in good, but not great physical condition (though with a very strong core) i suspect it will be more like 6-8 weeks, especially given the fact that I am having mesh installed for the repair which lengthens the recovery time somewhat.
Either way, i'll gladly continue posting regular updates in this thread to let you know how it all progresses, and what my doctor says. he is a cyclist himself who specializes in hernia operations for endurance athletes so he may have some interesting insights into recovery for trained individuals, and specific recommendations for cyclists.
I would consider it a success if i can at least get back on the trainer and soft pedal in 3-4 weeks. blood flow helps promote healing and such, as does mild muscle firing.