Really just whenever the mood strikes me (or a TBT event asks me to)—sometimes I'll keep an outfit for ages, other times I'll change only a play session or two later. A lot of it just depends on how much I like my current outfit, whether or not I want a different aesthetic, etc. (I also play New Horizons very intermittently nowadays, so I'm not counting the months where I don't play at all and thus never change clothes either )
I'll change my outfit every few weeks, and I match whatever the current season is. I kind of dress like I do in real life - simple, casual, and nothing flashy. I don't really like standing out in crowds so yeah. This picture pretty much sums up my style: