We are running a 400HP tuneup on a 500HP Nitrous Supply fogger system. I am ramping in from 200HP to 400HP over one second with an Edelbrock Nitrous controller.

How often should I get rebuild kits for the solenoids?

Anyone making kits for these solenoids, besides Nitrous Supply?

I know of two things that beat the seats up on the NOS solenoids, leaving bottle pressure on long after the run and NOC controllers that pulse the solenoids open and closed to ramp in power. It's easy to check them, just remove the plunger and look at the seats, you will see!

I am waiting for a call back from Nitrous Supply on repair kits.

It's been 3 years since they were new!

Long overdue. Just look at it as a necessary maintenance expense.
What gets overlooked beyond leaks is that the plunger wears into the seat over time and reduces the effective orifice area. Less nitrous gets through. This can sneak up on you with wondering why the system seems richer over time.