About my study visa

You need to either get a student visa for Singapore or study elsewhere.
No other solution!

How much time I can wait for apply for different college because I want to apply for Singapore only

Can I apply for different college for my study visa for Singapore

You can of course apply for admission at another institution and, if the accept you, apply for a student visa to study there.
But if the reason for rejection is you, and  not the institution, then the result will be the same again.
Therefore I recommend finding out the reason first and work on that!

I spoke to one of  friend he is studying in Singapore he told me your file everything is perfect but the problem is SOP As per my knowledge I think they didn’t prepare proper sop for me

I don't know what a SOP is ("Standard Operating Procedure" in my industry ...) and who prepared it for you for which purpose. But if that is the reason for rejection, can it be changed?

Meaning of SOP is ‘Statement of Purpose’
In this you have to give justification of your study your future plan why singapore this kind of I have to write.
Tell me one thing today I got my rejection so I was thinking I will wait for another 15-20 days next year I will apply for Visa from different college Is it good option?

So the SOP is something that you need to write to explain what your intentions and plans are?
Then of course it would not work if somebody else writes it for you!
Waiting or not waiting makes no difference, the only thing is if the reason for rejection is still present or not.