As a general rule, it correlates with how often you feed. Could be every six weeks, could be one a year. There's always exceptions though. I have an Avic avic that's molted only three times in a year. I have an Aphonopelma hentzi that molts twice a year. I have Psalmos that molt monthly.
Premolt can last just days up to months, and they won't eat. Usually. I've had some eat the day before molting.
My versi molted about once every two to three months. That was with being fed 2 to 3 times a month.
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