I use my decks to keep topics in mind for years until I choose to forget them. But these cards are things I always have memorized -- they're not something that I restudy when I need them.
25k cards hardly exceed your brain's capacity! That's about the same amount of information as 12,000 words in a second language -- plenty of people are bilingual.
Do I retain them all? Errrr... not really. My retention of cards I haven't seen for a while is about 85%. Further, 1,354 of the cards are "suspended" -- that is, I've gotten them wrong enough times that Anki won't show them to me again unless I un-suspend them.
The most important questions to ask any one who's using a flashcard system are:
- What do you get out of it? - What are your costs?
For me, I get years' worth of material I no longer need to look up and a good foundation in Spanish, even though I only travel every few years. But my costs are an hour a day, every day, trying to remember things that are just about to be forgotten.
Good luck!