Melbourne

Eel fillets with a nardoo crumb coating, fried in duck fat and served with bush pepper sauce. And finger lime tea sweetened with hakea nectar.

Lemon myrtle crocodile with Warrigal Greens and lillipilly jam followed by quandong clafoutis

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

I always keep stuff on hand to bake a basic cake and often have a cake mix on hand. You never know when that urge will arise!