IIRC, "stock" fuses are usually 15A. A lot of guys put a 20A fuse in. If you put a 30A fuse in there (not talking about "big" setups...more stock/mild-upgraded stuff)...you're defeating the purpose. Most setups don't pull more than 20A. A SAW hooked up to a car battery might need a bigger one...
I personally didn't run fuses in a number of my rifles...but I'm into them for testing/fiddling with new parts that there's a lot of preventative maintenance going on. Now most of my rifles have a MOSFET in them. The Triggermaster III mosfets have an automatic resettable fuse built in, can't remember what it blows at though.
I've given the option of having a fuse to a number of customers...most decline. There's A LOT of guys out there who don't even know where to find the fuse in their rifle...and don't know what it's for.