Days one to seven, while the water is still leaving
A new slab is at its most active in the week after the crew packs up. The mix went down carrying more water than the cement has any use for, and as that surplus works its way out, the slab gets very slightly smaller. The joint cut into the surface on the first day is the line that pull is supposed to release along. Our answer on why concrete cracks takes that mechanism apart properly, so all you need here is the outcome, which is a fine line sitting in a cut joint, sometimes before the crew has even been gone a full day.
A crack that early is not a sign anything was rushed. It is the quickest evidence you will ever get that the joint was cut in the right place and at the right time, because the shrinkage took the line it was offered instead of picking its own.
Weeks two to four, when most people actually notice it
Plenty of hairline cracks are never spotted on the day they open. Fresh concrete is dark while it still holds moisture near the surface, and it lightens as that moisture leaves, so a thin line can sit in a joint for a week or two before it has enough contrast to catch your eye. You are also on the slab more by then, walking it, sweeping it off, easing a car onto it. More trips across it is simply more chances to look down.
This is where most of the phone calls land, and it is worth saying plainly: the crack you noticed in week three very likely opened in week one. Noticing it late does not make it new, and it does not mean the slab is coming apart on some schedule. A tight line in a joint in this stretch is a slab that needs nothing from you.
The first winter, which is the only test that tells you much
By about the twenty eight day mark the slab is near full strength and the shrinkage that caused your crack is largely finished. What has not been tested yet is the ground underneath. At this elevation a winter is not one cold snap, it is freeze and thaw over and over across months, and the soil under a slab swells and settles through the whole of it.
A hairline crack in a joint goes through that winter and comes out the other side the same crack. It can read a hair wider in deep cold, because concrete contracts as the temperature drops and a joint is exactly where that shows, and it closes back up when the weather turns. The thing to check in the spring is not width, it is whether the two sides are still flush with each other. Level after a winter is a slab sitting on ground that was prepared right.
What to keep off the surface through that first winter, salt included, belongs to the first thirty days guide rather than to this page. It is a short read and it is worth doing before the first cold week.
Month twelve, and what should not have changed
A year out you are checking three things against your memory of month one. The line is the same width it was. The two sides are still flush when you run a hand across them. And the crack is still in the joint, not running off the end of it across open concrete.
Three for three and you have a finished crack. Shrinkage happens once. Once it has happened, the crack it left behind has no reason to keep going anywhere. A slab that has held its first summer and its first winter with the same fine line in the same joint is not quietly failing on you, it is done moving and saying so.
Why a crack at three weeks is not evidence of a bad pour
The worry sitting under this question is usually that a crack arriving late means something went wrong on pour day and is only now surfacing. Shrinkage does not run on a calendar. A slab poured into a hot dry wind gives its water up fast and can crack in the first day or two. The same slab poured in cool damp weather takes its time about it, and the identical line can turn up weeks later.
So the arrival date tells you about the weather that week, not about the work. What tells you about the work is the pattern: where the crack runs, whether one side has dropped, and whether it is still opening a year on. Those checks belong to our answer on structural cracks, and that is the page to go read if any of the three describe what you are standing over.
If none of them do, you are allowed to leave it alone. A tight line in a joint on a driveway does not need filling, sealing, or another thought. Call us if you want a second opinion on what you are seeing. We would rather look at a photo and tell you it is nothing than have you study the same crack every time you pull in all winter.