How much to feed?

I just adopted 2 adult male rats from the shelter and I'm sure how much I should be feeding them. I had rats years ago when I was little, but don't exactly remember the logistics :)

They're happy with Oxbow Regal Rat (though I find it odd to feed rats food that smells like dog kibble). But how much should 2 big guys get everyday/few days? I don't know how old they are because the shelter says someone just left them there (they were labeled as strays). I found the bowl completely empty tonight and although I figured they were just hoarding it in their bedding (I've seen them hide some), when I refilled it they both went pretty crazy.

I've only had them since Thursday so I'm still getting up to speed--any advice is welcome!