I have used Diafine for many decades, and can confirm that it really will last that long if properly cared for.
One important point...
As stated in the instructions, DO NOT use a pre-soak, regardless of recommendations for any particular film.
The critical concern is contamination.
If you even suspect that any of Solution B has gotten into Solution A, throw everything out and mix a fresh batch.
Use separate utensils (storage containers, graduates, stirring rods, funnels, etc) for Solution A and for Solution B.
DO NOT use those for any other purpose. Rinse everything immediately after use and set them aside to dry.
BTW, you'll need a small graduate for each developer solution, probably in the 25ml to 50ml range.
Mix each powder in its own graduate and store in clean bottles.
Make sure the Solution A bottle is filled to the brim. This is important for replenishment, as you'll see below.
Use clean marbles if necessary to fill the Solution A bottle. Solution B does not need to be completely full.
Mix a small additional set of stock solutions (1 quart of each) and store these in separate bottles (need not be full).
These will be used up in exactly equal volumes. Fresh batches can be mixed when needed.
Replenishment is very simple. You replace the lost volume of A with fresh, and add an identical volume of fresh B to its bottle.
When replenished as described below there's no limit to the storage life nor to the number of rolls it can develop.
When finished developing, return working Solution A to its storage bottle.
Add the amount of fresh Stock A required to fill the bottle of working Solution A, carefully measuring that amount.
Now return about half of Solution B to its bottle. Add some of Stock Solution B, the same volume as for Solution A,
and fill the rest of the way with the remaining working Solution B. Discard any excess. This half and half method
insures reasonable mixing of the solutions, although with pouring things back and forth that's really not a concern.
Discoloration of Solution A with use is of no concern. It's the film's anti-halation dye dissolving, which is perfectly normal.
You may also see some discoloration of Solution B; again of no concern. The color will vary depending on what film(s) you use.
As you've probably already noted, time and temperature are rather irrelevant for Diafine.
The temperature can be anything from 70°F to 85°F (21°C to 29.5°C).
Time in each solution should be at least 3 minutes, and no longer than 5 minutes.
There's no difference in results between those two times. I use 5 minutes since it gives me time for other tasks.
Agitation should be VERY GENTLE for 5 seconds at the start and 5 seconds each minute.