You can turn your extension upside down and see what is the max input current it can handle. Then with the information gathered, see how much (A) does the input current of your power supply and the rest of the items have. For example, at 230V my Aurum Gold 700 has an input current ampere (A) of 5A or below, and assuming that the extension can only handle a max load of 13A, you have 8A to support the rest of the devices.

Though the best load for a power supply would be within the range of 40 ~ 60%, 85% should still be within its limit. A good power supply will probably do the job well while a lower quality power supply may give out quicker than it should.

And yes, some PCs do reach to 90 ~ 100% load but that depends on what parts are you using. If you are still using a single core or a low-mid end dual core processor this days with a modern operating system and et cetera then the chances of it being loaded close to maximum load or maximum load is higher than people who have a fast-dual core / a quad core.

Anyway, as for how long a PC would stay at 90% ~ 100% load, that is impossible to tell since it depends on what is the user doing and when is he going to end the task that is taking up a lot of the processing power.