Good question, and a smart one to ask so early in your career. If you get proficient in tracking time spent, then you can better forecast how long projects will take, the more projects that you do, the more accurately you will become and estimating your time. This is valuable information to the money people and it can help you make cases for all sorts of things like faster design tools, scaling project scope, or even hiring more designers.
The answer really is “it depends”, and some of the things it depends on are the type of product and the scope of the project.
Ambiguous design criteria can be a blessing and a curse, if it is too ambiguous you may start to feel like you are designing in circles and spending more time concepting than you probably need to be, if you are designing for people who say “I’ll know it when I see it”, then expect to waste lots of time. When design criteria is overly detailed and defined then you may start to feel like there’s no room for creativity and exploration, however you will spend less time. So, think about how project scope and criteria is defined, I myself prefer to be handed a business problem to be solved and a given set of parameters and good customer research (or do the research myself, which I would also track time on).
If you aren’t doing so already, I would recommend that you start tracking your time for all tasks, that includes meetings and time spent on revisions and design changes. I might also recommend marking your revision and design change hours with who requested them. This way, if there is an individual or group that chronically requests an inordinate amount of design changes you will have data to make a case that they are a resource drain, again, this sort of data can help make decisions on a process change or hiring more design resources rather than just saying so-and-so wastes tons of your time, you’ll have stats to back that up and hold some folks accountable.
For example, you might only spend 12 hours on some solid concepts, but end up spending 60 hours on revisions, that might raise some flags.
There’s all sorts of time tracking tools out there, I don’t have any good recommendations, but whatever you use to do this I would recommend making it a habit and stay on top of it.
If you can tell us more about the types of products you’re designing, then there’s likely someone here on Core77 that can give more advice than “it depends”