A conservative "transition" time frame you can expect is up to a year. This is the time that the hooves adjust to not having shoes on. Some horses adjust much quicker. Some horses always need boots on rocks, depending on a lot of things, like what kind of ground they are on the most, etc. Your trimmer probably (should have) explained this to you. The longer the horse has been in shoes, the longer time it may take.
Concrete can highlight the lack of sufficient sole having built up (magnifies pain if your horse has a rotated coffin bone, or weak hoof walls) since there's not enough sole material to cushion the bones in the hoof from the ground. In dirt, hooves do sink into it, even if it seems like hard ground, a little and the toe digs in the most as the hoof breaks over in motion.
Could be your horse needs more time to build sole up. Or it could be showing a need for the trimmer to take the toe back a little more. Concrete will create a longer breakover point than dirt or turf. Won't hurt to ask your trimmer about it and have him/her watch what you are talking about. Usually if it's sole issues, the boots help, but your horse may even need foam pad inserts for a while to make him more comfortable. The inserts are cheap, btw.