It realy depends on what type of system you try using them on
say system 6-7 need more than one becouse they are inneficient, a 4fd on a sys 11 will give you 6-9 months backup
8fd on a sys 7 boardset will give you 2-3 months
a 1.5 on a bally -35 will give you 6 months, or over a year with a 4fd charge time varies leave the machine on say 2 hours youll be pretty much loaded

Every IC is going to be different. I want to say i noticed that PCD5101P uses way less current than some of the other 5101 types. PCD5101P would last a year or more in theory if im doing the math right.

Memory caps are a good option to get the batteries off of the board. I use CR2032 lithium in a holder only because it is cheaper. In a Bally a cr2032 should last like 10 years and i can do it for under a buck.

My NGG came with a 1F one from previous owner. It only lasted 8-10 days at a time.. Garbage, looks like it was undersized.

Just replaced with NVRAM instead, prefer that approach

If you want help doing the actual calculation about how long it would last, I can teach you how to do it, just PM me.

You only need to take one measurement for the calc.

I put a 1.5F in Williams system 11b and a gottlieb system 80. So far so good. I never monitored to see how long settlng last. I usually turn games on for an hour or so during the week.
Yeah it is pain I guess to have to remember to turn on all the time but a really good cheap option to having no batteries on the board at all.

Ed at GPE (great plains electronics) has them. Next time your placing an order pick up a couple they are worth it.

See this thread for more info: https://pinside.com/pinball/forum/topic/memory-capacitor-fans

I get 10-14 days on System 6 with a 1.5F cap.

I found you needed 4F on a system 11 to get similar results.

I even have a 4F in a WPC era game (I500 and T2). Settings and scores are saved for over 2 weeks but the clock runs slow. WPC draws much more "off" current due to the ASIC keeping the time.

Some people suggested using a resistor to limit the current when charging a totally dead cap this large. On most of my games, I just jumpered the blocking diode with wire, I have not experienced any negative effects. On my I500 I jumpered the diode with a 270 ohm resistor, it seems to work just fine. I have my machines on for a couple hours per week (at least) so this solution works great for me.

I made some rough current draw measurements on WPC-S in the thread linked above. Also posted some low voltages I tested where the RAM still held.

I got just over a year with my Xenon with with one of these.
Actual span varies widely depending on specific machine and 5101 memory used.