I do a full change of the substrate once a month as well. i dont use aspen as a substrate though. it's cheap, but both me and my gf have allergies. the aspen for whatever reason causes my allergies to flare up. i use coconut bark instead. it's a tad bit more expensive, but just like aspen it's easy to spot clean. it also can hold moisture without getting moldy. aspen can get moldy.
i was given a hamster and he was on aspen. i had to change it out a week after i got him, the aspen was soaked and molding in the corners of the tank. although i dont know when the last time the person changed the aspen. he's now on carefresh. thats also a decent substrate thats super absorbent but it's not very attractive. hamsters produce more waste than a snake though.