I am not sure what joshuanapoli was referring to, but I can give an Idea from my normal job (3D printing is just to relax from that )
As soon as you want to create income from machinery some random failure / machinery break down has to be considered.
Saying that I appreciate that you are already looking into the right direction with your question!! (where is the thumbs up emo???)
Even though I have no interest at all to charge any body for 3D prints (I have a totally different daily job) I found your question interesting enough to start some “research”. The numbers below I am sure are not really representative but I believe they can give an impression in ball parks that should be considered for a business .
It seems that even today 3D printing is still in its beginnings and there for difficult some reasonable numbers. Never the less here what I have figured:
Average MTBF (Mean Time Before Fault / Failure) for a 3D FFF Printer seems to be apr. 1800 hours.
This MTBF could be longer if there would not be spurious failures of machines. about 15% of all reports I found had a machine failure after it was working fine, but before it reached 500 hours.
Again: I know this is not a proper research, but more a quick data collection and from there an best guess Statistics live and die with the information basis and below is how I generated mine:
With the help of Google I found 487 considerable statements for working time before first major repair.
Major repair I called everything that required spare parts which are not available without order / shipping time.
If I use the above mentioned data I would calculate the following:
Printer usage 24/7 gives 8760 hours. Considering the apr. 1800 hours MTBF you have 4.86… let us say 5 machine failures per year.
In other words: Every 2.4 month the printer fails and needs spare parts that you might not have on stock.
From here I am going to be very pessimistic
I am not sure for 3D printers, but for a rule of thumb I would use 20% of new price / year as spare parts.
Considering that delivery times for spare parts can easily be 4 weeks a second printer should be considered to keep business running.
For all that disagree to my numbers: You might be totally right
All the Best