Full strength is a later milestone than it looks like
A slab looks done the day the crew leaves. It is hard underfoot, it holds a finish, and within a day or two you can walk on it without leaving a mark. None of that means the concrete is done curing. Concrete cures through a chemical reaction between the cement and water that keeps building strength for weeks, not through drying the way a puddle dries, and the visible signs of being finished show up well before the strength curve is finished climbing.
That is the fork worth understanding: usable and full strength are two different lines on the same curve, and usable comes first by a wide margin.
Why 28 days is the number people quote
The strength a mix is designed to reach, the psi-rating on your estimate, is conventionally measured at 28 days. That is not an arbitrary round number, it is the industry's standard test age, and it is close to the point where the strength gain has slowed enough that the concrete is doing most of what it is going to do. Concrete does keep gaining a little strength past that point, but the curve has flattened out considerably by then.
A high-early-mix is built to get to a usable strength faster than a standard mix, which matters for a schedule that needs a slab back in service quickly. It still follows the same general shape of curve, just shifted earlier.
The practical milestones that come before it
Most residential concrete is fine for foot traffic in 24 to 48 hours after the pour, and ready for a car on a driveway after about a week. Both of those numbers sit well inside the first month, long before the slab has reached the strength its mix design is rated for. Heavier loads, trucks, trailers, campers, and equipment, are the ones that should wait longer, generally until the slab is close to that 28 day mark, unless it was built thicker for that specific load ahead of time.
Treat the 24 to 48 hour and seven day numbers as what they are: typical ranges for ordinary residential use, not a promise that the slab has finished curing. Weather, mix, and thickness all shift the exact timing for your pour, and we give you guidance specific to your slab when we walk it with you.
What slows the whole curve down
Cold weather is the biggest factor. The chemical reaction that cures concrete slows down as temperature drops, so a slab poured in winter is on a longer road to the same strength number than one poured in summer, even with the same mix. That is one reason winter pours get scheduled and protected around the forecast rather than treated the same as a summer pour.