The answer to this question depends on what substrate you are working over. If you are installing adhesives over soft surfaces like plywood, then a trowel can last quite a while. If you are installing over a concrete or hard, abrasive surface, then a trowel may last about 2,000 square feet before the notches wear down. An installer may think that this is a good thing because his coverage will get better per gallon of adhesive, but in the end this is not a good situation as there may not be enough adhesive on the floor for proper bonding. Some trowels may last longer, but this is always something to check before getting started on any given flooring project: Make sure that your trowel is in good condition and will deliver the right amount of adhesive required for the project.