That depends on what the computer problem is. But let's give you a typical scenario...
Let's say your computer has suddenly become slow to start-up, you get one or two error messages, your Wi-Fi doesn't work any more or is erratic, and you get a load of intrusive advertising.
That kind of computer repair would take about 1 to 1½ hours of manual work followed by automated work taking 8 to 12 hours (overnight) to complete; and provided that no spare parts are required, you would get the computer back the next day at 5pm after testing and the cost would be about 1½ - 2 hours labour.
Much preferred is a 48 hour window of remedial opportunity because if a repair is rushed, something lurking may be overlooked and we'll get you coming back complaining instead of singing our praises. A 48hr repair window is especially important with intermittent faults, which, by their very nature only happen, "sometimes" . Intermittent computer faults can be tricky to troubleshoot quickly.
WHAT CAN HAPPEN WHEN A COMPUTER REPAIR IS HURRIED
A real example; one such rushed computer fix was a while-you-wait computer repair and there was no time to scan and double-check the work done and so when the customer returned home, the original problem re-surfaced somewhat. Not entirely, but if that computer had been thoroughly scanned before leaving the workshop, that oversight would have been eliminated.
Computer viruses and malware are sneaky things and can often hide and disguise themselves. A slow and lengthy scan (or scans) often pay dividends in the long run. At least you'll know that every file, old or new, large or small, has been tested and is safe to reside on your hard drive.