Can I be on unpaid study leave for 6-8 months on H1B visa?

In general, to maintain your legal H-1B status, you need to be working in the position for which the H-1B was approved for the Employer who filed the H-1B on your behalf. Regular vacation, genuine sick or personal leave in general does not cause you to loose your status. However, a 6-8 month study leave could in my opinion be an issue. There is no specific regulation directly on this issue and USCIS will decide on a case by case basis. The F-1 visa status is more appropriate for students, not an H-1B on leave. USCIS could take the position that studying full time is in fact a violation of your H-1B status - studying is allowed only if it is incidental to your maintaining your H-1B status. The fact that the employer is willing to grant you leave without pay is not relevant to whether you are maintaining your status - at most it may relieve the employer of the obligation to pay your the required wages for the duration of your H-1B. Remember, “unpaid” time is generally not allowed and this could be seen as just a ploy to get around the “no benching” rules. Finally, when you apply for a change to F-1 status, you will need to show (among other thiings) that you have maintained your legal H-1B status up to the date that USCIS receives your application. I suggest you contact an attorney who is experienced in handling H-1b and related issues.