I am planning to join SBL classes next year in January, and giving P7 exam in March God willing.
So can you please advise me when is the best sitting to do SBL exam? June 2019 sitting (within 3 months of study) or September 2019 (6 months of study) sitting?
It can be done in three months, but depends on what else you have to do at the same time, such as working or another paper. If you can concentrate in SBL, and have time to try lots of questions properly, three months, heading to March should be OK.
You might want to keep options open by leaving it as long as possible to see how you get on before enrolling for the exam (28/1/2019 standard, 4/2/2019 late entry).