Since I got 940c about 3 years ago, I'm on my third 78. THe first one
that came originally lasted (IIRC) 6 refills before I had to buy a new
one. The second one was a little dissapointing at 3 and my current
cart is running off refill #4 and still looking good.
940c does not use 45 but it uses 15 which is virtually identical to 45
except for resolution. I've gotten one to last 11 refills and it was
rattling loudly when I dropped it off to be recycled. (free ream of
paper from Office Depot from used ink cart and toner)
YMMV, depends on how often you use the printer, the quality of ink
used, if it was refilled before it goes empty or not, and if you do
anything to clean the head in between refills. Letting printer run
while it's out of ink can cause stubborn clog and eventually burn the
nozzles out so refilling it before it goes empty helps a lot.