How does the length of the ad time work?

[Johnny Doomo]

When you change the the dropdown box that denotes that amount of time the ad will be posted for 15,30,60 and 90 days. What is actually changing? Do the ads disappear? Do they stop showing up on searches or on peoples watch lists? I'm having trouble seeing what changing this time affects on the placed ad. Can somebody please help me understand what it's doing and what it affects?

Thanks.

[Andy R]

Also, How do we change the lengths if I want to offer really long terms like 2 years?

Thansk