100% is the soft limit, the hard limit is a little trickier to pin down. This other questionThis other question, at 170%, is the highest I've ever heard.
Instead of asking how much you can bend the limits, I'd recommend starting to plan how you can get your org under the storage limit (or purchase more storage from Salesforce).
Salesforce is being nice, and not immediately cutting your org off. Don't abuse that or take it for granted. Take the time to get your org under control, or it will come back to bite you (in the form of sudden failure that will probably take days to resolve).