Fliprrr - you will likely get more precise answers on the
Jayco Hybrids Forum.
I had a 17Z with similar specs and never used the FW tank for anything more than water while on the road or parked at my storage site. That said based on the rate at which the wife and I fill our grey and black tanks, and given an almost 30 gal FW tank, 2 or maybe 3 days would be the limit if you had really quick navy showers and were very careful when washing dishes and brushing teeth, etc. water usage varies wildly between people so asking on forums would not give any exact answers for you.