Sorry don't mean to hi-jack here.....but I'm about to put pellets on line tonight.....and wondered what everyone recommended to start out with......ei, I was thinking of starting with 250ml on a 130 gal system

Is there an optimal ml per gal for starting out with?

Start with 1/3 recommended amount, wait a few months add some more and so on until you are up to what you want. I run 1000 ml of pellets in my MR1 until there are only a few inches left then dump in another 1000 ml.

How long does it take till you need to refill?

For me around 3 - 4 months or so