There are several things to remember when estimating this task as Janet pointed out. What is the soil like (compacted, clay, loose), clay soil is harder to make an initial cut but sometimes the loose soil is harder to make a nice clean cut. How deep is the new edge going to be? How much debris is going to need to be cleaned up? If you have a lot of debris not only is it going to take you longer to clean up the edge, it is going to take more time to clean up and haul that debris away. You said you are not making a new edge design but if you were that will be added time. Is there edging in place? I have several sites that have steel edging, I have found it easiest to follow that edge with a knife on my hands and knees, which becomes very time consuming. I also don't recommend power edger's, if you're not sure where cable and sprinkler lines/heads are, you're sure to hit one or two with that! Then there is more time to repair that. I would say Janet's hour for 75 linear feet is a good starting point but if this is the first time you're edging this site I would estimate it a bit higher just to be safe. Once you've done the job you will have a base number to use in the future for that site, always remembering that conditions may change and that number may not stay the same.