Mikrotik/Radius server keep alive time?

I run DMA and use the prepaid generated card system.

Most of my clients accept this but a few have the complaints of having to log in every time. Has anyone figured out a way that the client can log in once and not bother for the rest of there “time period” with logging in again?

I know I can do the mikrotik access list but it would be to much work/time for the amount of clients i have.

Thanks in advance.