Muscle strains are often described in grades, and people use them to guess how long they will be out. The grades help, but return times vary a lot within each grade.
A simplified overview:
Grade 1: minor fibre damage, mild pain, little loss of strength; often days to a few weeks
Grade 2: partial tear with clearer strength loss and pain; often several weeks
Grade 3: complete or near-complete tear; may need specialist assessment and, in some cases, surgery
Newer systems, such as the British Athletics classification, also consider whether the tendon inside the muscle is involved, which tends to mean a longer recovery
A useful rule: being pain-free walking or jogging is not the same as being ready to sprint. Before returning, most rehab plans include full strength, reaching high-speed running in training, and sport-specific drills without pain.
Skipping the high-speed running stage is a common route to a re-injury.