Should be more than 5bil realistically.
However, wont be higher than Zoro's final one.
On the other hand, when I think about it, Zoro's bounty never needs to surpass Mihawk's.
Beating Mihawk will make it clear that Zoro's bounty is above whatever Mihawk's is.
Even if Zoro's never gets updated from 1.111 which says 4 times that Zoro is #1.