How long will my data stay online?
As long as a file is paid for, it stays forever. Files uploaded to Skynet are governed by file contracts on the Sia network, and as long as those file contracts are paid for and valid, files stay online.
Uploading from your own Sia node
When you upload a file to Skynet using your own Sia node, you're paying for the file contracts to begin with. As long as you open Sia every couple of months to refresh those contracts, the file will stay online.
Uploading through a Skynet webportal
A Skynet webportal is a special Sia node that has already formed contracts with every available host. Since webportals pay their own bills and for the files uploaded to them, they will likely have their own policies on how many and what types of files you can upload.
When someone pins a file
No matter who uploads a file to Skynet, anyone else can pin it. When they pin the file, they are saying "I want to keep this file online even if no one else does". As long as any single user is pinning a file (which means they are also paying for it), it will remain online.