Posted By Paul Duell
You're never legally required to buy "computer chairs" - you're legally required to risk assess and provide appropriate equipment. The fact that you're already considering how long the computers will be used, suggests you're already into the risk assessment process whether you realised it or not!

You ought to consider -
Do your trainees have any pre-existing back conditions?
Do the "ordinary" chairs have reasonable back support, are they reasonably comfortable, and will they put most people fairly close to the right height?
How long will the trainees spend in the chairs (which you've already thought of)?
What sort of computer use are they doing? If all they're doing is watching something happen on the screen with an occasional mouse click, you can be a bit more liberal than if they're doing speed typing training!

Hope this helps to point you in the right direction!