How deep does my window need to be for an inside mount?

It depends on the product, and sellers publish two numbers: a "minimum" depth (the blind mounts but protrudes slightly) and a "flush" depth (it sits fully recessed for the clean look). As rough, confirm-with-the-seller guidance, a slim cellular shade wants around 3/4", a roller closer to 2", a roman about 1.5", and wood or faux-wood blinds 2" or more. Measure the flat, unobstructed depth at the shallowest point of your opening — mind cranks, alarm sensors, and existing hardware that eat into it. Compare your measured depth to both the minimum and flush figures on the product page. If the frame is too shallow for even the minimum, an outside mount is the right answer rather than forcing a fit.