The official channels (Windows Blog, Twitter pages, Conversations blog) are all saying "coming months" or "Q4 2014" - Q4 started this month, and will continue until the end of the year, so essentially any time between now and December 31st is when the first phones are believed to receive it (but there's no 100% guarantee this will happen). If Cyan is anything to go by, after that the update would likely have a staggered release to cover all of the different regional variants of each phone. Carriers/networks can also hold this up for their own phones as they may wish to approve the update first, so generally unlocked phones (CV's - Country Variants) will get the update first. This causes the possibility of some phones not getting it until early 2015 (although I would think Microsoft would be keen to avoid repeating the long rollout Cyan had, so they'd push it out fairly quick).
So basically, it's hard to say when your 635 will receive it. As it is a popular and newer model that is still being heavily advertised around the world, I presume it would receive it faster than, say, my older and less-advertised 1020, but how long it will take will depend on whether your phone is locked or not, who it's locked to if it is, and what region you are in.