The general rule of thumb is 75% of 28-day strength in 7 days. Most concrete mixtures reach their design strength well before 28 days due to the fact that any particular batch of concrete will be mixed such that its 28-day strength is higher than the design strength by a few 10s of %s. This is due to the fact that failing a compressive strength test for any one test or any 3 averaged consecutive tests (guidelines are set forth for failure criteria in ACI) requires remedial action in general.