As much as you need, or as much as you can apply before it kicks, whichever is less.

I find 3 fluid oz a comfortable amount to work with at a time. Other folks like more.

Laszlo

I agree pretty much with Laszlo, but will expand on his comment a bit. 

Be aware that the amount you can safely mix will vary with temperature, with whether you're using "fast" or "slow" epoxy, with how quickly you're working, and with whether your working container for the mixed epoxy is a wide, shallow container, or a narrow, deep one -- wide and shallow being best, as the epoxy will take longer to "kick." You'll be surprised, and distressed, at how quickly it can kick a deep container (e.g., a plastic cup), as the heat of the reaction accumulates and speeds the curing -- a plastic cup holding too much mixed epoxy can actually get hot enough from the curing process to melt.

I found that depending on conditions, I usually did best mixing up no more than 4 oz, and more often the 3 oz that Laszlo suggests, except that when I was spreading a lot of epoxy quickly I could use more, provided I was using a large, flat pan to hold it.

Another advantage of mixing smaller quantities is that you'll have less wastage. After all, epoxy is probably the most expensive component of your boat.

Here is my first epoxy attempt. It was about 6 oz and a horrific failure. I've stuck to 3 oz. since then.