I don't know if this is possible to implement, but I think it would be better to base the devouring of marines on health rather than time?
My reason is that if you have about 10hp, and you get devoured, I think it would be much better to be killed nigh immediately than have to wait however many aeons to die. It's also illogical to assume that a marine with 10hp and no armour will survive just as long in the stomach of an onos as a fully suited HA.
Feel free to point out any flaws.