You are ultimately in control of how long your posting will be visible. The default will be 30 days, but you will be notified when it is expiring and be able to extend it if you choose.
Yes. You must be a registered user to create listings. Registration is free.
Choose the category that best fits, it isn't an exact science as many categories blend into others and that is okay.
No. You can browse and search without registering. If you want to create a job posting, you will need to register. Registration is FREE and will save you time, as the site will remember various preferences for you.