It doesn't really matter - just fit them into your schedule. Sometimes I double at 11am and 5pm, sometimes it's 6am and 8pm, sometimes it's even 1pm and 4pm (that one isn't ideal).
If you are only building to 50mpw, you don't need to double that much. Get some 6-8 mile runs in there, and an 8-10 miler once a week. Most runs should be at an easy pace (5k pace + 2min), and don't go any faster than 5k pace + 1min for any sustained period.
Throw some strides (ST) in 3-4 days a week, however. Once you're up to 50mpw, a normal week might look like this:
M - 6w/ST
T - 3 + 5w/ST
W - 6
T - 4 + 4w/ST
F - 6
S - 8 (a little faster than most runs)
S - 3 + 5w/ ST