The money invested in ELSS is locked for a minimum of 3 years from the investment date and cannot be withdrawn under any circumstances. This is the least amount of mandatory lock-in period for any tax-saving instrument including tax-saving FDs, life insurance, PPF etc.
Though you can invest any amount in ELSS, one can avail tax benefit of up to Rs. 46,800 by investing up to Rs. 1.5 Lakhs under Section 80 C.