the phrase "mesgul musun?" means are you busy? But i have been told this refers to someone being busy and you want their attention. How do you ask someone about their job/work and ask them if they are busy?

how do you ask someone who is on holiday or work how long they will be here..."how long?" and "how long are you here for?"

cok tesekkeur ederim

holiday: tatil / izin

I think these two questions are what you look for:

tatilde misin?  izinde misin?

They both mean "are in holiday / vacation?"

About timing;

İznin ne kadar? How long is your vacation?

or asking ending time:

İznin ne zaman bitiyor? / Tatilin ne zaman bitiyor?

or asking when to start working:

Ne zaman işe başlıyorsun?

or some old type alternative:

Ne zaman iş başı yapıyorsun?

It is also used to know if the one whom you are talking to is doing something other than listening to you, and yes, this may mean you want their attention.

cok tesekkeur ederim to you both.

so can i use the term mesgul musun when asking someone about their work?

Hangi işle meşgulsünüz? ´What is your occupation?´

For "what is your job/occupation", I think you can try one of examples from here:

http://tureng.com/search/what%20is%20your%20occupation

As a learner, for "are you busy" I would try with one of the following (it´s more like "are you busy at your work" ?). Frankly, these are only guesses (my Turkish is still based on many guesses ) - so I don´t recommend using these sentences unless someone confirms that they are ok

İşinde çok meşgul müsün musun?

İşin çok yoğun mu?

meşgul is a funny word. As a loanword it does not follow vowel harmony but it only shows in the declined forms, even in the question particle that follows it:

                                   Meşgul müsün?

Good to know Thanks for this, I´ve just fixed the first sentence in my attempt.

Tomac, thanks for your suggestions which are correct. I have just spoken to a turkish man and he says both are good.

Thanks to everyone else who left messages.