Welcome to the forum. The quantity is always a guess, but Stefan gives a nice rate. I give my youngster's 2x week and my addults 1x every 2-3 weeks. But I give them a bit more then 20% of it's body weight.
Since you ask about the quantity, I wonder if you know what you can give it? Fish is a nice one, but if you give fish be certain that you know about the thiaminase problem. For this I would suggest to have a look at this
thread, there is a discussion about it starting rom page 4. It's quite important that you know about this, because otherwise you'll snake might get to little vitamine B1 and die from it. Other things are mice, earthworms ( not the red one's), cat food with fish taste, steak (can be a bit more of a challenge), ... It's always nice to have some variation in the diet.
Good luck with your new one! Post some picture's if you can, it's always nice to see.