Howdy, Master Mi! Besides calculating time-based effect durations, you can also use this calculation to place studio monitors in your space.
First things first: the speed of sound through a given medium depends on the medium's density (and temperature). At room temperature (20C or 68F), the speed of sound through the air is 343 meters/second (about 1,125 feet/second), slower at lower temperatures and faster at higher temps. To calculate space in time-based effects like reverb and delay, that's 343 millimeters per millisecond (343 mm/ms) or about 1.125 feet per millisecond (1.12533 ft/ms).
Next, sound waves radiate from the source -- almost perfectly spherical in lower frequencies and more directionally as the frequencies climb. A spherical room with a radius of about 11.25 feet will have early reflections (i.e. bounce off the walls) at 10ms. Of course, no room is spherical; most are rectangular prisms or combinations of 3D shapes, so you'd need to measure the distance from a given point in a room to all the faces (walls, floor, ceiling) to determine the travel time for the sound to reach that point from the wall and vice versa. Because higher-frequency sounds have smaller wavelengths, they lose energy faster than lower-frequency sounds. For especially large rooms like your cathedral example, it'll be necessary to roll the high frequencies off in your reverbs and/or delays for realism (and lower frequencies for mix balance). I like rolling off at 360Hz (high pass/low cut) and 3600Hz (low pass/high cut) as starting points but play around with these to suit your production.
Regardless of the other dimensions, the average height for human males is about 1.77 meters (5.8 feet), while human females stand at about 1.63 meters (5.35 feet) so the time for a sound to travel from human-ish height to the floor is about 4.75-5.1 ms (1630 to 1770/343 or 5.35 to 5.8/1.125). But unless the sound source is facing the ground, most of this first reflection is likely not getting through your HPF. For the distances to other surfaces, convert the distance to either millimeters or feet and divide by either 343 (mm/ms) or 1.125 (ft/ms).
It'll be up to you to decide how big you want this cathedral to be and calculate accordingly. You can also fudge those delay timings to simulate a colder or warmer room -- e.g. at 0C or 32F (freezing point of water), the speed of sound is about 331 m/s (1086 ft/s), or 331 mm/ms (1.086 ft/ms), so the time to the floor is about 4.9-5.3 ms. Not much slower for the first reflection, but the difference is more noticeable for further surfaces.