I think focus is the most important thing when you image and the most overlooked. It depends on your scope and how big your CFZ is. With my old Q if temperature drops 0.5 degree I'll be out. With my C11 it's a lot more forgiving. With my CN-212 it's sharp enough to show small changes even at long FL.
So focus goes out quickly at the beginning of the evening when you start your imaging session then stays somewhat linear throughout the night provided there aren't rapid temperature shifts.
I do the initial focus with a Bahtinov. It's quick and easy and good enough. Keeping focus while imaging is the challenge.
Practically when some scope cool they tend to contract or expand so you'd need to rack the focuser out or in to compensate. If you use an OAG it's very easy to see your guide star and other faint stars in your guider fov. You know you're close to focus when you can see other fainter stars in the same field. So that gives you a good indication of where you sit. You focus and more stars appear or disappear.
Another way to check where you're at is to view the FWHM of your subs with a tool such as CCD Inspector as they are saved to your HD. If there's a trend towards growing values then unless the seeing or your guiding has turned bad you know you're slowly going out of focus and you can compensate accordingly while you're imaging before it gets too bad.
I know there are tools such as Focusmax or others that will use a V curve to find perfect focus but what's the difference doing it with a Bahtinov mask if you're going to be out within the next 30min or hour? You'll have to spend time doing another V curve and waste imaging time. Unless you're remote doing it manually is quicker.
I'm now looking into a system called
Lacerta done by the bloke who did the ONAG. That might be a good all round solution to non stop focusing if it works as advertised.