It really depends on what difficulty you play at, what your victory condition is, and what your goals are. For peaceful deity though:
If all you're trying to do is win, turn 50 benchmarks aren't particularly important. As long as you're not dead, you should be able to win the game. I wouldn't worry much about your culture/science output on T50 as a lot of that has to do with the city states you meet and their quests. That being said, you should be close to picking up political philosophy - that might mean having to build a couple monuments in the early game. In terms of expansion, I would aim to have 3-4 cities down. I don't build many military units early unless I have to. I'll usually open with a scout, pick up a slinger later to get the boost for archery, and I'll eventually get three so I can upgrade them to archers for the machinery eureka, but this is often all the military units I have. I'll have significantly more if the AI starts sending units toward my territory, or I get horse barbs spawning units.
Turn 100 benchmarks are far more important imo. I would look to have at least 7-8 cities founded by turn 100. I actually aim for 12 provided I have the room to expand. In my experience, I can almost always get eight cities down which is plenty enough to win any victory condition. Science/culture output depends on the victory condition. I'm most familiar with science victory benchmarks and would say that your in good shape if you're making 40+ science and 40+ culture per turn. If you care about winning quickly, I find that 100 science and 100 culture per turn on turn 100 usually puts me in line for around a turn 200 victory. I pretty much never do much better than this, but better players definitely can. Ideally, you should be close to your tier two government if you haven't already got it. In the tech tree, you should be close to (or have already researched) apprenticeship, machinery, and education. My personal turn 100 goals are to have my industrial zones (three of them) placed, and to have finished chopping out the Kilwa Kisiwani (IMO the best wonder in the game and one that I try to always build). I would also look to be making 100+ gold per turn or faith per turn depending on what kind of economy you are running. In terms of units, again I only build units if I have to. If I can lock down friendships with my neighbors, I don't build any units outside the ones that could possibly give me eurekas/inspirations. Even then I pass on some of them (kill a unit with a spearmen is a common one).
If you're in the middle of the pack in terms of science/culture by turn 100, you shouldn't have any problem racing past the AI in the next 50-100 turns. Even if you're near the bottom, the game is very winnable because the AI is so awful. In my last game, Korea was making 170 science per turn on turn 100, and was somehow making less than 300 per turn when the game finished around turn 220. Meanwhile I went from about 90 science per turn to close to 2k.