Why a fixed number is the wrong question
Ten minutes of focused, honestly-scored recall on twenty due cards can do more for your memory than forty minutes of half-attentive review where you rate everything easy just to get through the pile faster. Time spent is a weak proxy for what actually matters, which is whether you genuinely attempted retrieval before checking each answer.
A more useful target is a review session that ends when quality drops, not when a timer or a fixed card count is reached.
A reasonable starting range
For most students maintaining an active deck, 15 to 30 minutes of daily review is a reasonable starting point, adjusted up or down based on how many cards are genuinely due and how dense the material is. Dense material—detailed medical facts, exact legal definitions, multi-step calculations—will need more time per card than simple vocabulary.
If review is regularly taking much longer than that and still growing, it is usually a sign that too many new cards are being added relative to what reviews can keep up with, not that you need to study for longer.
Signs you should stop
Guessing instead of retrieving, rating every card the same regardless of how it actually went, or reading the answer before you have genuinely attempted one are all signs that continuing will not add much. Stopping a slightly shorter, higher-quality session and returning tomorrow is usually a better trade than pushing through a long, low-quality one.
An adaptive scheduler like Cueable's FSRS-based system also helps here indirectly—because it concentrates review on the cards actually at risk of being forgotten, a well-maintained deck naturally keeps daily review time more predictable than a flat, unfiltered review queue would.