There is no ideal time in terms of difficulty brother, it remains the same
The ideal time is the only one when you are prepared for the challenge.
From my personal experience,
I appeared on 9th September and I got
L 6 R 5.5 W 5.5 S 6
Overall 6
Then I decided to take test again and I appeared in 2nd December and I got
L 7 R 6 W 6 S 7
Overall 6.5
There is no difference between IDP and British Council, it's a rumor, some says IDP is easy and some says British Council is easy, but trust me, if you are well prepared, you will achieve your desired score.
Practice smartly
All the best