That’s a really good question but I’m not sure that I have a simple answer for you. My first thought is that I think it is important that you only stay in the pose for as long as you are comfortable. That is, you are not feeling strained or coming out of alignment and you are able to breathe deeply. This may be a few seconds or a few minutes depending upon where you are in your headstand practice and will vary depending upon how you feel on a given day.

A recommended length of time (according to Yoga Journal – I’ve linked to the article) is 10 seconds for a beginner while gradually adding 5-10 seconds every few days until you reach 3 minutes. Then you can hold at 3 minutes for a bit and gradually build to 5. Personally, I would estimate my headstand is usually under a minute.

I don’t think there is any limit to the frequency. In many traditions (such as Sivananda) the headstand is practiced routinely and is part of the core set of asanas. So barring any physical limitations or ailments, I see no reason why you couldn’t practice on a daily basis.

Hope this helps!