When you take a machine to a computer repair shop, you probably would like some idea of how involved the job is going to be. No technician can tell you for sure without conducting a diagnostic, but there are a few factors that usually signal whether a system will be problematic.
Complexity of the Issue
Sorting out a single problem with a computer is typically not a terrible task. A technician can usually do a test run with a machine if it's operational and then run some diagnostics. If the machine won't boot, the technician can start looking at issues with the power buttons, motherboard, and power supply.
Where things can get challenging is if there are multiple problems. This is especially troublesome when there's a cascade of failures. For example, if a faulty motherboard goes too long without a replacement, you might start to see data corruption or storage failures. The computer repair technician then has to solve the problem with the motherboard before they can determine if the storage is also going bad.
Location in the System
While the modular nature of most modern computers makes it easier to break a machine down, it is still hard work. The easiest work involves the components that are most accessible. A technician can usually just open a case to remove and replace memory modules, for example. The main time constraints for that project are getting the parts and testing the system with the new parts in.
Conversely, removing and replacing the motherboard gets pretty close to a full teardown. The technician has to remove components from the case and unplug the power systems, too. If they're trying to reuse the processor, they'll have to move it and its cooling system from the old board to the new one.
Availability of Components
The availability of computer components is highly variable. For example, tracking down a high-demand item like a top-tier video card could take weeks and cost thousands of dollars. That's especially the case if the customer doesn't want to substitute the component for a different product. On the flip side, most modern memory modules are available within days.
Components for older systems, especially vintage ones, can be harder to track down. If the components are out of production, the technician may have to find replacements on the used market.
Once a technician has handled the hardware issues, they have to get the systems running. This often means doing basic setup work, especially installing device drivers. For most popular components, this is easy and fast. It can take some work with less common ones, though.