I have an actual number for you, if that helps (vs. all the speculation)

One 1-hour ride on Strava (yesterday) resulted in 175kb of data usage on my Nexus 4 (they graph it out in Settings->Data Usage->Strava). My bet is that around half of that will scale linearly with ride duration, so expect more data to go when you have more data to send... There's surely some handshaking and version checking in that 175kb that will happen no matter how long your ride is, and not get bigger. I'd bet you'll add somewhere around 90kb per hour after the first one. That part is speculation though I'll post back after a longer ride sometime.

To prevent uploading before you get home to wifi, you might need to go into airplane mode, then finish your ride and save it, then manually kill Strava so it won't try to upload in the background, then go back out of airplane mode. When you get home, start Strava and it should sync quickly on its own. If not, browse to the ride, and hit the refresh button. There may be a simpler way to do this... but I don't have to worry about it.

FWIW: I'm running the T-Mobile unlimited data/text no-contract plan for $30/month. It. Kicks. Ass. (b/c of the value) Coverage is not outstanding, but generally is fine. The best part is saving around $575/year vs. Verizon. I only get 100minutes of talk, but I'm running Google Voice for all outgoing, so if someone chatty calls me, I call them right back. Additional minutes are only $0.10 each, so I could use an extra 400 minutes and still be below my Verizon monthly cost. My phone was only $300, so it's paid for in the first two months (vs. the $150 I'd pay for an S3 or similar), and it's GSM, so if I travel I can just get a monthly sim card in country and be in business for about another $30.