How to decide on the length of a showreel
Your showreel should be no longer than 3 minutes. Casting Agent's tend to have a lot of showreels to go through when making casting decisions and a longer showreel often gets dumped if it goes on for much longer than 3 minutes. (Although show-casing all of your acting projects Casting Agent's tend to be incredibly busy people and do not have the time to reflect on your entire acting career... sorry!
If you are able to afford it do get a professional showreel done. They are expensive but go a long way in portraying you as a professional actor. Having a showreel that you have knocked up with your home video camera will come across as a bit tatty.... (however, some casting agents will agree that it is better than no showreel at all).
Make sure you edit in the scenes that show your acting ability at its best, if you are unsure, get the unbiased & honest opinions from your friends, family & colleagues. (What you may think was wonderful may appear weak to others, so don't "shoot the messenger", take the criticism in a positive way). Sadly you may have to bite the bullet and drop some scenes that you are really proud of.
If you have many scenes, for example, where you are playing the part of an aggressive character you may want to choose the one character that gives the best performance of an aggressive person, a sad person a terrified person ... and so on.
If you decide to add a music track to your showreel, try to ensure it is not too dominant, it should not be allowed to distract the viewer's attention from the task on hand.
Start the showreel with an introduction so the Casting Agents know who it is they are watching. You may even add stills (Such as your headshot with your name, website and contact details).
You should include your contact details at the end of the showreel as this will not only make it easier for Casting Agents to contact you, but so they may remember you for any future projects.