Agree with Ed T,
The toughest operation I've seen on lift trucks is at bulk brake truck terminals that use LP solid (cushion tire lift trucks). These terminals work 24 hrs/day 7days/week 365days(or 366) per year. At two different companies they averaged 5.5 hour per 8 shift.

It would be hard to answer your question
1.) You indicate high hour usage (15 out of 24 is significant when you deduct lunch breaks 2 or 3), morning & afternoon breaks (say 15 min./brake) x number of shifts, work shift change time 15 minutes per change, battery exchange time.
2.) The units need planned maintenance service at about every 170 operating hours based on a normal & clean operation at 15 hours per day the units be done for PM service every 20 days.
3.) When you say abuse what does that mean - abuse can be a simple as breaking a flashing light or running the unit off the shipping dock & any thing in between. If operator abuse what does management do to correct this or they just do nothing & let the operators run the show.
4.) How many work shift are there 2 or 3
5.) What type of dust are you talking about? - carbon dust, gypsum dust, grain dust, granite cutting dust, West Texas or Oklahoma dust storm dust or just dirt type dust because no one sweeps or scrubs the floors.
I would suspect even with this type of info - it would be hard to project % down time.