How much do you feed an 18 month old python?

If you let them strike at the prey each time - the answer is simple - keep feeding until they don't strike anymore. If you use 'pop another in' after the first strike, then this is more difficult - if you over feed then they will probably regurgitate (so at least you know for next time). Depends on the weight of the snake - my 18 month olds weigh about 1Kg and eat 3-4 large mice a week.

Hope that helps