First off, it's perfect to say 你弹吉他多长时间了? "时间" means "time". So it's equivalent to say "How much time have you played guitar?" 久 means long. So the perfect translation of "...how long..." is actually "...多久..." "...how much time..." is "...多长时间..." Just like in English, you can also ask "how many years have you been playing guitar." (你弹吉他多少年了？) Note that you used "have been playing guitar" which is the present perfect continuous tense. For perfect translation, you need to add some words in it. But Chinese speaking in daily life is not that strict. People will understand it. If you want the detail about the tense compatible with English, that's whole another story.

How long have you been playing guitar? 你弹吉他多长时间了? This sentence can be translated word for word as "you play the guitar how long time?". Given that there is tense in Chinese, we have to say "弹多长时间 （play…how long time)" instead of "been playing".

how long ... means "多久"，we translete "多长时间" in english is "how many years ... " for exemple : How long have you been learning english? 你学英语多久了？ "久" in Chinese means a very long moment, so the perfect translation is "how long..." How many years have you been playing football ? 你踢多长时间足球了？or 你踢多少年足球了？ how many years ... usually we translate in Chinese is " 多少年", but it means also "how much time" for signifie period of time. I hope I have answered your question ! Have a nice day !