I see Common Blues in all months from May to November.
First seen usually end of May, but one year (2008) on the 7th May and two other years in ten in mid-May.
The second emergence in August is much larger, sometimes a hundredfold.
The big blue females ar more noticeable earlier in the year and I have found they are a greater % of the females.
Lots of competiton of three species of blues commonly found on chalkhill: Adonis, Chalkhill and Common Blue. On the short sward I think the Adonis are likely to be quite dominant even when outnumbered by Chalkhills. Common Blues are more numerous in the longer sward meadows but the big blue females are not seen there so much.
My expereince. I got some pics but they are not sorted. I'll find one.

This one was taken in late 26 September 2006. Note the Devil's Bit Scabious which gives a clue to its lateness.