Jen, it's not an exact science by any means.
It's a risk to fix any specific dates until after the visa has been issued. Definitely don't buy a non-refundable, non-flexible air-ticket to the UK until you have actually received your visa.

UKBA stipulates that visa application can be made up to a maximum 3 months of your stated intended date of travel.
In other words, the visa validity start date cannot be more than 3 months advanced of the date of application and you're right, in that within these general parameters UKBA decide the actual start date.

Current Settlement Visa processing times for December 2013 show:-

5 days = 6%
10 days = 25%
15 days = 40%
40 days = 78%
60 days = 100%

(5 days = 1 week)

Visa's leading to settlement (spouse/fiancee etc) will have a validity duration of 33 months from the visa start date.
You can enter UK at any time after the visa start date but not before.
The key is to be sure you enter and still have visa validity of 30 months otherwise it'll mean an additional costly visa just to meet the 30 month qualifying residential
period.

Based on December stats if you submitted the application say March 22 you could expect a decision at anytime up to June 22
I guess on average that end of May time would seem most likely.

Highly unpredictable