External bearing cartridge BB for me, so no need to worry in regards to servicing.

Hubs, I probably do them once every 9-12 months. I do check them regularly though, and I'll do them if they feel a bit dry or gritty, or if I've recently ridden in heavy rain.

Same with the headset.

If they're fine for now, and cost is a concern, they probably could go for a bit longer while you save up the funds. But in the end, it's preventative maintenance, and peace of mind, knowing they've been done and not having to guess if they're okay or not.