Truly, apple crisps are best eaten the same day they are baked and are especially good warm out of the oven. As time goes on, the topping tends to absorb the fruit juices and get softer. If you want to store the crisp, be sure to cool it completely before covering. Any residual heat will turn to steam under cover and that will make the topping soggy. As a side note, I've found that recipes for crumbles tend to retain their crispness just a bit longer.