Well the answer appears to be up to 6 weeks, unless you are desperate to use them now!
I had two award tickets LHR-HNL and return which I booked several months ago. When the recent UK to US fare sale came along I booked a revenue ticket with Air New Zealand and cancelled the award tickets.
Now, three weeks late I am still waiting for the miles to be recredited. Speaking with United this morning they tell me that I may have to wait for a further 4 weeks as ‘Mileage Plus are snowed under’.
In the ‘old’ days, before the United/Continental merger, re-credits were instant with United.