I would assume that you're talking about a Sulcata because this is Sulcata Central, so the amount of squash would depend on the size of the tortoise. I give Bob squash a couple of times a week.
For a small tortoise a couple of spoonfuls a week would be alright...not all Sulcata keepers think squash is alright to feed regularly, I think it's good for them. For a baby I cook it, but I serve Bob's raw...I think it's good for his beak.
Yellow squash is pretty soft so I just cut it up into discs half an inch thick or so, they eat the crap outta it. Its a good thing to dust with your supplaments, If you were going to use something like butternut or acorn squash or something then you may need to cook it for a few, for the little ones. I also will give them zucchini sometimes....but usually only about once a week or so.
~Aaron