I would think use, and perhaps most importantly, the pads materials. If it doesn’t do well getting cleaned, I would do it less often. If it tolerates washing fine, then as often as you like.

I wash it about every 3rd or 4th time that I wash the sheets. I replace them so rarely, I wait til they are falling apart.

I wash the mattress pad 1X per month. I haven’t replaced it in maybe 8 years. I bought an item that was pricey but seems to be worth the cost; it’s holding up well and shows no signs of wear.