Hi Bashi
Pleas link below
https://www.gov.uk/tier-4-general-visa/extend-your-visa
How long it takes
A decision will be made:within 8 weeks for online applicationsusually on the same day if you use the premium serviceYou’ll be contacted if your application is complex and will take longer, for example because:your supporting documents need to be verifiedyou need to attend an interviewof your personal circumstances (eg if you have a criminal conviction)
They say 8 week anything longer you should contact them.
But home office are unpredictable! !
Hope this helps