How much of the rated capacity of a lithium ion battery can I discharge?
I you have a "90 Ah" battery pack, it doesn't necessary mean it has all of it.

First, the battery capacity is rated at a certain discharge current as [it should be] specified by manufacturer. If you discharge it at higher current, the battery won't have all 90 Ah to deliver. 

Second, the actual battery capacity varies depending on charge conditions. The "90 Ah" is likely quoted if it is charged to some specific voltage level and charge current. If you charge the battery under different conditions, it may not achieve the "90 Ah" design benchmark.

In most cases the advertised capacity of no-name batteries can be obtained only under very "relaxing" charge conditions (small current over long time), and in most cases this number is highly inflated.

Third, the end of discharge is determined by the BMS. A good BMS will cut the battery off if its voltage drops below certain limit, so the pack will only "deliver" whatever was stored in it.

In sum, no, there is no guarantee that your battery pack will "safely" deliver 100% of 90Ah of advertised capacity.