See ACI 530.1-11 Specification for Masonry Structures part 3.5A: "Place grout within 1 1/2 hr from introducing water in the mixture and prior to initial set."
Well the setting time also affected by w/c ratio. Now a days, the initial setting time of cement can be greatly increased by introducing admixtures. I don't think 90 mins period grout should be rejected but that is another case if it is in your TS.Simple way to confirm is to test it at your site's material testing laboratory to find the initial setting time.