Roughly how long did each "stage" (splining, polishing, etc) take you for the example animation you shared with us? How long would you expect a professional to typically take with an animation of similar length/complexity?
That's not quite a straight forward question to answer. When you are learning, do not worry about how long it takes you. If you work in a logical way and practice, the speed will come - I promise. Just focus on doing your best and ignore the clock. If it is no longer fun, or you have reached you skill limit, move on.
The time frame you have in the "real world" will affect how long you have on each stage.
TV quality animators roughly have to output about 5 seconds of finished animation a day (6-8 hours of work) and Feature quality is roughly 5 seconds of finished animation a week (30-40 hours).
But it really does depend on the action too. Shots with multiple characters or full body shots are much harder than simple stuff.
You might think that the fast action shots are harder than a character just standing around not doing much but it's actually to opposite. The longer the action the longer it takes to animate. A fast action might be difficult to figure out the mechanics etc but there are less frames in the animation so it can be easier/faster.
My one take away from all this is, just learn and have fun. You're practicing, not perfecting.