Melbourne

I wash the kids' drink bottles nightly, because they get dirty from them having dirty hands etc. They're grots, basically.

Mine, I wash probably less than weekly, but more than fortnightly.

I wash mine weekly. It's metal and there's no straw so it's basically a thermos. I only have water in mine.

It partly depends on what it is made of and what sort of opening it has. Plastic, particularly soft plastic, is a surface that algae and things can easily cling to so yucky things build up easily. You probably want to wash those out thoroughly on a more regular basis, although I wouldn't say you have to do it daily unless it you do things like add lemon slices or have a stye that any contents can touch your mouth. Stainless steel, hard plastics and glass have surfaces that stay clean a lot longer, and if you are regularly changing the water in it there should not be a problem leaving it a long time between cleans - especially if you are filling with tap water as that still contains residual chlorine.

The lid itself is another matter as you are putting that in/against your mouth. That bit should be cleaned very regularly, probably a basic clean every day, with a thorough clean of any inside surfaces every week - if you have a style with a straw make sure you get a brush that can clean inside it.

every single time it's empty

i use a bottle brush and soap and water then rinse

I have more than one bottle