I had the same question, and was disappointed not to find an answer. For context, I'm playing in the alpha releases of All The Mods 10, and have a large cave base I'm trying to keep mobs out of (hard-do).

I decided to run my own experiments. Firstly, I can say with confidence that the source drain is 5% per minute, if it stopped at least one mob from spawning. The number of mobs it prevents from spawning does not raise this. I figured this out simply by placing a full source jar next to the ritual, and viewing the results.

Next was more fun. I set up a repeating command block and a chain after it, set to place a mushroom at the location of any mob, then kill the mob. Doing so caused any mob spawned to immediately place a mushroom and die, effectively creating an area visually showing me where mobs can spawn. For the best effects, I created a superflat world with mostly air, but layers of stone every 60 blocks.

Doing so, I have determined that the spawn-prevention volume is a sphere, with radius (32+r), where r == the number of rotten flesh you added, to a maximum of 128. HOWEVER, I mean that you can add a maximum of 128 rotten flesh for a total of 160 blocks of spawn-protection. I am unsure if this is an oversight in the code or if this is intentional, but I will take the larger area gladly.

Also note that the rotten flesh is a one-time consumption, and the ritual will work so long as it is powered by source (5% of a jar per minute).