That's tough to answer.. with "heavy doses" of hydrocodone I guess I'd say.. 10 days to 2 weeks. You're definitely going to have to allow some time for your tolerance to go down, and even with that you still might not find kratom as enjoyable as before :/
Not sure what you consider heavy but I take oxy daily at 130mg+ and I can take kratom the next day and sort of feel it. By day 3 off of oxy I can feel the kratom very well.
Depends on how long ago (time wise) was your last hydro dose/usage , average hydro dosage(mg) and then your kratom dosage(g). just a shot in the dark, though, i'd say wait a solid hour after your kratom dosage to see what's up.