1. A pun is a play on words, they are normally not that funny and are often based on words that sound similar. Time and thyme (an herb), for example. No pun intended is something people usually say to draw attention to a pun, although it technically means that they made the pun by accident.
So, while cooking, I could tell someone that I have a lot of thyme on my hands, no pun intended (or I can say pun intended if the pun was intentional and planned). Since thyme is an herb, this can literally be true. However, since thyme is pronounced the same as time, there is no way for the speaker to know that I'm not using the expression "time on my hands" unless they are looking at my hands. Likewise, there is a common pun that says that people who wear two watches (which by the way looks quite ridiculous) are trying to have more time on their hands.
Likewise, "to raise the roof" is a dance move sometimes used at parties. However, it can also literally mean to raise a roof during construction. To say someone's days are numbered means they don't have long left with whatever it is that they are doing. It can also mean that the days are literally numbered, like in a calendar.
Therefore, some other examples of puns include...
How do construction workers party? They raise the roof.
Every calendar's days are numbered.
2. Yes it is. However, an "eternity" instead of "a long time" is more dramatic, so you may accused of whining/being over-dramatic if you use it too much. Every once and a while though, it can work to get your point across. You can also say something is taking eons and that will get the same idea across.