Depends on the sealent you have, temperature, how well sealed your system is, perhaps how many punctures you’ve had.
I use Orange Seal Endurance, which should last twice as long as standard sealents. I check it after 3 months then every month and a half or something. It normally lasts 6 months before a top up is needed.
I used to shake the wheel and listen to check the level, until I had some sealent that turned to water a couple of times. The shake and listen still produced a decent sound, even though the sealent had stopped working. So now I try to find the effort to remove the valve cores and use a dipstick to make sure there is still actual sealent inside.
As for cleaning, I sometimes peel the worst off before installing a tyre, especially around the bead, but it doesn’t really matter.