Hello taxpayer,
Good to know you are availing the benefit of QRMP scheme.
In your case, if you are able to self assess your liability precisely and pay it by monthly due dates then no interest would be charged. Otherwise interest @ 18% start accruing from day following monthly due date.
However I would suggest you to go for 35% challan payment mode because here you don't need to do any manual computations and in case your payment is lesser than actual liability for the month then you have time till quarterly GSTR3B due date to pay the same without interest.
Feel free to reach out to me through telephonic mode for detailed discussion.
Best Regards,