If you have a long-term residence permit (which requires a minimum of eight years of uninterrupted residence in Turkey), or if you have worked in Turkey for at least five years, without interruption, you may apply for an unlimited work permit. The unlimited work permit never expires, and you can change employers without having to apply for a new work permit. You will enjoy the same rights as a holder of a long-term residence permit.
My Take on This: While the law says you can get an unlimited work permit after living in Turkey for eight years without having a long-term residence permit (which implies that you can then go directly to the Ministry of Labor and Social Security and apply for one), it is unlikely the Ministry of Labor and Social Security is going to give you one if you don't already have a long-term residence permit. This is because the unlimited work permit gives you all of the same rights you would have with a long-term residence permit, which requires documentation beyond the usual short-term residence permit (criminal records check, proof of perpetual income, proof you have not received social assistance in Turkey, and additional scrutiny).
The Göç İdaresi Genel Müdürlüğü (Directorate General of Migration Management, or DGMM), already handles these things during long-term residence permit applications. So you will probably have to receive your long-term residence permit first, and after you receive it, then apply for your unlimited work permit.
Even though you can work for any employer with an unlimited work permit, you must still report to the Ministry of Labor and Social Security what company you're working for, and if you change employers or if the address where you're working changes. You must report such changes within 15 business days or you will pay a fine.
Interruptions in Residence
To accumulate the five years of working or eight years of residence necessary to apply for an unlimited work permit, you must not be outside of Turkey for more than 180 days at a time during your work or residence period. If you are, this will be considered an interruption of residence or work, and when you return to Turkey, you will have to start accumulating your work or residence time all over again.
If your work permit expires (for example, if you stop working for an employer) and you don't get another job with another work permit within six months, but continue to live in Turkey with a residence permit, this is also considered an interruption of your work period (but not your residence period).
Exceptions: Annual leaves, absences due to occupational accidents or disease, other health issues or temporary physical disability, or periods of unemployment for which you receive unemployment insurance are not counted toward your time outside of Turkey. If this is your case, be sure to obtain documentary evidence to demonstrate to the Ministry of Labor and Social Security that your absence was due to one of these exceptions.
Your unlimited work permit may be canceled if your long-term residence permit is canceled. It may also be canceled if the reported circumstances under which you received the work permit are found to be false.
Unlimited work permits are issued according to law Number 6552, Article 6 (amended by article 22 on September 11, 2014).
Useful Telephone Numbers
You can contact the Ministry of Labor and Social Security using their national customer service number, which, from within Turkey, is 170. From outside of Turkey, call +90 216 170 1122. If you will be working at a University, contact the Council of Higher Education, at +90 312 298 7000.