I have about 300 cards in my deck right now, and I've had this deck for about 6 months. I try to do a few cards each morning without forcing myself to complete all cards that are due (although often I do). This is because I'm trying to build the habit into my work flow and I find if I give myself the option to quit after 3 cards if I'm really busy that it's better than losing the habit altogether for a month.

I want to warn you that I don't think this number (300 in a 6 month time span) very accurately represents how many cards I add when I find something interesting. I'm a PhD student and the last 6 months I've been mostly tweaking a program and writing my thesis (so doing things that I don't get a lot of new knowledge to add into the system). So most of those 300 cards are from short spurts of reading interesting books, rather than what it would be if I were in the midst of researching something new. (I also usually make multiple cards for a single image that I add, asking the question in different ways that emphasize different things I think are important)

For context: I have had larger decks in the past, multiple deck systems, etc etc., but all previous attempts were eventually abandoned because of something like: it takes a long time to add new equations or transcribe text from a book by hand -> therefore I slowly add fewer and fewer cards, and the ones I do are of lower quality because I would cut corners to save time -> the deck becomes less useful because I'm just not adding things that are important -> as the deck becomes less relevant I stop studying it -> once I miss a month of studying catching up seems useless.