I would think this would depend on the poo size and freqency relative to the tank being used. If you have a pretty big snake that gives you really wet squishy poo like a big ol' rat snake in a 20L you might have to clean it more frequently than a baby ball in a 40 gal.
Also the substrate probably makes a difference. Paper towel might need being replaced more frequently than cedar bark chips or aspen. I like the cedar chips myself, because it looks natural, smells great, and works like kitty litter. They clump together around poo and urates.
Do you do any light cleaning in between as well? I remove poo and urates as soon as I see them, and I check every day.
I'm not saying once a month is a bad idea, but I've had success with my snakes and I only do a total tank clean every 3 months.
Also
This is essentially my approach. The other option is use a plastic milk jug with the top cut off (or like object) to scoop out as much of the mulch as you can instead of the shop vac. remove the bits of leftover mulch with a regular vac or pick it out by hand.Personally, I recommend a shop vac, paper towels, and a spray bottle with a bleach solution. Vacuum it out well, spray the bleach/cleaning solution around and use the paper towels to wipe it out. I also leave it for about 30 minutes after I wipe it down to make sure that any fumes have dissipated and anything I missed has evaporated. That way you don't risk your UTH or dropping/damaging the tank.