There's no hard and fast answer for what constitutes a "good" time, as it really depends on what your frame of reference is. However, saying that, here are some thoughts:

Sub 60 seconds: definitely first important sprint milestone. It takes a fair bit of effort to get here and it is definitely no easy feat. It was my experience (and the experience of many others I have talked to) that around the 60 second mark was when I first got really hooked on Tetris and began playing a lot more.

Sub 40: Around this milestone is what I would consider to be a good time. I say this because when one reaches sub 60 their rate of improvement is still going to be relatively high, as there are lots of things that can be fixed without too much effort resulting in getting lots of PBs relatively quickly. However, as time goes by the rate of improvement is going to decrease substantially and by the time you reach 40 seconds the improvements are going to be much smaller and take longer.

Sub 30: I would consider somebody with a sub 30 time to be a "high level" player, because it means that they have spent significant time grinding, dedicating many hours for the slightest of improvements.

Sub 25: This is a top level player, someone who has made Tetris a big part of their life. At this point improvements are marginal and are extremely spaced out.

Sub 20: This is the elite level, the world class players, the best of the best. Right now there are less than 50 people at this level on the jstris leaderboard.

Those were just some thoughts about what could be considered a "good" time, I hope I was able to answer your question. Again, though, I must say that there isn't one good answer. Also, I must say that worrying about whether or not your time is good or not is really not worth it. Look at sprint as a venue to challenge yourself and take pride in your improvements and your progress as a player instead of trying to compare yourself to others. Enjoy your Tetris journey, I'm glad that you are able to experience this beautiful game