Both are correct.

*WHEN* if you have more questions or want to improve your skills feel free to book a lesson at any time of the day Speak to you soon

Technically correct would be **in which** I am supposed.... But people say when or where all the time. I would say there is a slight preference for "when" because you are talking about time.

Thank you for your help, Ben Corey.

I would prefer to hear "where". Maybe it's a North American thing, but "reaching a point" (whether in reference to space or, in this case, time) makes "where" jump to mind. "When" has an awkward sound to my ears, though I can see the inclination to want to use it since you're referring to a time. "In which" does definitely work, but would be slightly less natural in a conversational setting.