I think others will be able to comment on when would be the best time to buy your ticket. From reading the forums and website, I can make the following suggestions:

(1) To upgrade with miles, your ticket will need to be at least $1,200 roundtrip. I think you can upgrade one way if your one-way ticket is at least $600. There will need to be availability in whatever bucket is used for Envoy upgrades. Availability in Envoy does not necessarily imply availability in that bucket.

(2) You can also upgrade with GoEnvoy for $500 beginning 30 hours before your flight, regardless of the original cost of your ticket, assuming there are open seats in Envoy. This would probably be a cheaper option if you are going to need to buy 22k miles on top of a $600 ticket.

(3) I am not sure what destination will be least expensive at that point -- to get into Envoy you're probably best off on an A330 which has 29 seats for sale as opposed to 17 on the 762s by next summer, which means LHR, CDG, FRA, FCO, or MAD. My guess is that MAD will be your best bet, but others who have actually upgraded will be more able to help you. The bonus is that flights from MAD within Europe are pretty inexpensive.

Hope this helps.