How many days of leave? Under what conditions?
Basically, central government rules apply, plus some special provisions for teaching faculty. Government rules provide 12 days casual leave. Casual leave can not be carried over. Casual leave cannot be taken if the length of absence (including prefixed/suffixed days) exceeds five days; for absence exceeding five days and for availing LTC, one must take earned leave.
In IISc, the period of May, June, July (90 days) is called vacation. For every three days you work, you get 1 earned leave. Thus, if you do not take vacation at all, you will get 30 days as earned leave. Faculty can choose to take all the 90 days as vacation and not get
any earned leave. So if you work through without any vacation, you will get 30 days of earned leave (like non-teaching staff). Earned leave can be carried over subject to a maximum accumulation of 300 days. Any travel (including attending conferences abroad/local) during this period will be considered as availing vacation.
There is medical leave which can be availed with the submission of a medical certificate. The duration of medical leave permissible depends on the medical conditions. Then, there is half-pay leave, leave under extraordinary circumstances and even something called leave not due (taking leave when you dont have any to your credit).
You have to apply to the department chairman for casual or earned leave or vacation. All records are studiously maintained.
Whenever you go on official work (presentation to the funding agency, giving a talk, attending a selection committee etc.), please send a note to the chairman of the department informing him of the same. This is considered as on duty or absence on academic work. No permission is required but the chairman should be informed. The chairman of the department informs the divisional chairman when he/she is traveling, the divisional chairman informs the director etc..so it applies to every faculty. This is important because if anything untoward happens when you are traveling, this letter is required to claim medical reimbursement and insurance.
The teaching faculty are eligible to take a year of sabbatical upon completion of six years. Sabbatical can be used to visit a host academic or research institution or industry in India or abroad. There is a department wise cap of 33% of strength who can avail sabbatical at any given time. IISc will be paying full salary for first six months (without the Transport Allowance) and later six months without the dearness allowance (DA). You can retain your quarters during the sabbatical; Some allowances (like transport allowance) may not be given during this period. Technically, your host institution cannot give you a salary - but can grant you a living allowance. The official document on sabbatical rules are here.
There are two more kinds of leave where IISc will not pay your salary. One is "on lien." This is when a faculty becomes a director etc of another host institution. In this case, IISc will not pay the salary and other benefits but the host institution will pay the salary etc till the lien period ends. Then there is EOL - extra-ordinary leave. The faculty may spend at another institution without IISc paying the salary and other benefits. However, availing EOL means that period will not be counted against your service. Extraordinary leave and lien may also be availed against specific request and approval. For more details on leave, please see here.