You can't say how long a season lasts,
S1 lasts 6 months and I thought S2 only last 4 months.
So the average time of a season should be around 5 months indeed.
on average 4-6 months, generally you can time it with PvE patches. s2 came with i believe an ssc/tk nerf to attunements and encounters. s3 3 weeks after BT and most likely s4 2-3 weeks after sunwell. blizzard seems to go with giving PvE a head start so the hardcore guilds can get over the basic learning curve and gear distribution is somewhat even on both ends.
s3 was 11/07 if i remember? so i would expect 04/08-05/08 somewhere in there, which should allow 2.4 enough time on ptr and to be launched 2-3 weeks before s3 will start.