I've put over 10hrs on the bench without a prop on. Kept the revs under 5000. No issues here, good for debugging and modifications on the fly. You have the psru on, fly wheel, water pump, alt, so a bit of a load anyways.
There are some Mazda manuals on break in procedures in cars. Basically don't sit at one rpm for a long time. Everyone will give you their opinion on breaking procedures. Some say to start it up and drive it like you stole it, some say to baby it.
- Matt Boiteau