There is no limit as such. It is not really a first-come, first-served system since there isn't really any limit.
If I am not mistaken, the 15 billion $ amount is distributed as follows : 4 billion $ spent in the 2005-2006 fiscal year, and another 11.5 billion $ to start the current fiscal year. When there is an election, the appropriation of vote 5 is not important anymore. We appropriate the money without any prioritization.
We have an overall amount and various requests. We must appropriate the amount. We have done that in order to minimize the number of attestations that would be required. Some of the requests were for minimal amounts. Then, we used vote 5. However, there is no limit and there is no obligation to inform Parliament afterwards.