First, did you have an agreement in writing? If so how was it stated? Was there any mention of what would happen if it went over 5 hours?
Second, why do people charge hourly rates in this business? It’s either a full day rate or half day rate, where anything over 4-5 hours (depending on your half day definition) is then charged at a full day rate.
Get the client to sign it.
Third, if that’s not your agreement, contact the client and say you’re in danger of going overtime, ask if they wish to pay additional fees or shall you stop shooting at 5 hours. Then you’re covered.