Nephrolepis exaltata
Boston ferns dry out fast and punish it with crispy fronds. They want soil that stays evenly moist, plus humid air, which makes them one of the thirstier houseplants.
Short answer: for a Boston fern, with the starting settings below (an 8 inch pot with a drainage hole), check the soil about every 5 to 8 days in spring and summer and every 8 to 11 days in winter, and water when the top half inch of soil is just drying out. Give it a full drink until water runs out the bottom.
Your answer
Water when the soil reaches this point
How it shifts through the year
Water slowly until water runs out the drainage holes, then empty the saucer within an hour. The right amount depends on pot size, so go by drainage and how dry the soil is, not a fixed measure. Ferns want evenly moist soil, so soak until the whole root ball is damp.
More often than most houseplants, usually every few days to once a week depending on light and season. Check by touch.
Dry soil and dry air are the usual causes. Keep the soil evenly moist and move it away from heat sources.
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Related guides
Estimates use rules of thumb. Always confirm with your finger: push it about an inch into the soil before you water.