I'm not at all proud of this, but for a long while, like as in up until my mid-twenties, I was this way. I think for me depression was a big part of it, but I think nose blindness also play a big role. People (mostly family) would tell me that I reeked or that I looked grimy, so it's not like I didn't know, but I couldn't smell how bad I stunk. I couldn't feel how greasy and grungy I was. So even though I knew they were bothered by it, I didn't feel any urgency to do anything about it because I didn't see what the big deal was. When you wallow in your own filth for long enough, you really don't mind it.
As I got older, somehow I got my shit together all on my own and began showering regularly. After I had done this for a while, I was now able to smell my own stink if I went too long between showers. I don't even like the way my hair feels after 1-2 days now, if I don't wash it at least that frequently then I feel horrified to go out in public. I'd still say that you'll be okay if you don't bathe every single day (unless you regularly exert yourself physically or work outside), but anything past 3 days is seriously pushing it.
The only thing I'd change now is that I'd like to waste a lot less water. I tend to take 15-20 minute showers (because let's be honest, a hot shower is pretty relaxing), I'm not like those ultra efficient people that can fully bathe in under 5 minutes. But I'd like to be more environmentally conscientious about it if I can.