GST Liability

Your turn over is more than 1.5 cores? If yes then you need to file the GST return monthly and your out put tax liablity arise monthly.

if you follow mercantile accounting system, As per mercantile accounting system you need to raise invoice monthly.

Better to raise invoice monthly. 

It is ok to pay gst on a quarter basis if your turnover is below 1.5cr.

Yes. But you need to raise the invoice on monthly basis.

As per rule for issue of tax invoice section 31 :
[5] Subject to the provisions of clause [d] of sub-section [3], in case of continuous supply of services,--

[a] where the due date of payment is ascertainable from the contract, the invoice shall be issued on or before the due date of payment;

[b] where the due date of payment is not ascertainable from the contract, the invoice shall be issued before or at the time when the supplier of service receives the payment;

[c] where the payment is linked to the completion of an event, the invoice shall be issued on or before the date of completion of that EVENT.

So in your case the due date is ascertain able, as it is from contract.