If you see this, brush your teeth

And floss too. If it's the evening, that's a perfect time to do it. If it's the morning, great time as well. Middle of the day? You've probably had food, brush them teeth. If you've recently (but not immediately after, wait a bit, drink some water) eaten, that's a perfect time to brush. Especially if you've recently had something sugary (avoid sugary stuff, also like I said not too recently).

And if you can, and haven't recently, see a dentist.

Your teeth are precious, and hard to replace. Lack of proper dental care can in many ways amount to a tax on the poor, where a lack of proper preventative care can turn mouths into an unaffordably fixable mess. Get people in your community to take care of their teeth too.

There's really only a few situations where you shouldn't be brushing your teeth right now: you vomited recently (brushing after vomiting can degrade the protective coating on your teeth), or you brushed recently.

Not having the equipment for it on you is only an excuse this once, carrying that stuff with you is a good idea. You eat and drink throughout the day, so you should brush and floss throughout the day. And who knows when you'll need to be presentable? You don't want to barrage your comrades, friends, partners, or "path to promotion" (your bosses) with bad breath.