I was in a similar situation 18 months ago, and also bid 'up' to WB (767-400). They started paying me the day I got back at the -400 pay rate. It took 7 weeks to start training, partly because you do the first 85% of the ER course before 'converting' to the -400 and aligning the two sims in the proper order was a scheduling challenge. Obviously, if you go back to the 330/350/777, that particular issue wouldn't apply. I got the distinct impression 7 weeks was WAY longer than they wanted to wait.
Though you would need at least a little time to do the academics, you should at least expect to start training pretty quick, but enjoy extra time should you get it - though I would recommend making smart use of any time you get to study up. You'll also have to do the QCQ's for all the time you were out, in your 'new' aircraft (much of which will make little sense since you are not qual'd yet...) I presume they still have a Worldwide Ops LMS course you will need to do (for pay).
They won't necessarily rush you the iPad, so you might bring that up ASAP with Pilot Assist. Brush up on ARCOS, and a few other things that have changed since you've been out. You will also have the opportunity to adjust your insurances and such. DPMA has a pretty good 'return' checklist. Congrats, you'll love the WB life. Welcome Back!