Hi there. They are both very similar but there are some slight differences. Your 2 example sentences, 1) How long have you been having guitar lessons? 2)How long have you had guitar lessons? are both grammatically correct.The basic rule though is that we use the continuous form (How long have you been doing something) when you focus on the process: you're interested in how long something has been going on. So you'd rather say How long have you been having guitar lessons?) and the present perfect form if you're more interested in the quantity - how many (e.g. how many guitar lessons have you had so far?). Hope it helps.

I’m not sure I’d use the second example – it doesn’t feel correct to me. If the person is still learning guitar now, then you would say ‘how long have you been having guitar lessons?’. If they had lessons in the past but aren’t having lessons any more, you’d say ‘how long did you have guitar lessons for?’ Hope that helps!

1) How long have you been having guitar lessons? 2)How long have you had guitar lessons? (I'm trying to detect the differences in use. Are they similar or not?) ---------------------------------------------------------------------------- To reply to your query in the simplest of terms; 1) How long have you been having guitar lessons? This can be used, if the process is still continuing, as in, the person is still continuing with his guitar lessons. 2)How long have you had guitar lessons? This is primarily used if the process has stopped. In this case, the person is no longer having guitar lessons. Hope all is clear now? =)