Torque Wrench required for trainer cassette installation?

How important is torque wrench on this?

It's not important at all. The cassette lockring just has to be tight enough to not come off as it's not doing anything other than keeping the cassette from sliding off - some cassette lockrings are even make of aluminum.

Use a long wrench and put a bit of force behind it and you'll be fine.

Having the exact torque generally isn’t very important. I recommend finding a way to torque it properly the first time you change a cassette so you get a feel for how roughly tight it should be. It’s hard to just say “oh yea, it should be decently tight” because everybody’s opinion of “tight enough” is different. People also tend to be pretty inaccurate when it comes to estimating weights and forces. A torque wrench borrowed from a bike shop (or even automobile shop) would be best, while a luggage scale would be adequate, and even a bathroom scale might work in a pinch.

40Nm = 29ft•lb as you said, so that would be roughly 30 pounds applied to the end of a 12” wrench. You can do the math if you have a different length wrench than that.

Not mentioned so far - a torque wrench has two functions. One is to ensure enough torque is used to make the nut grip as required by the application. The second is to ensure you don't overtighten/strip a thread. If you are an adult, and are using a 1 foot/12 inch/30 cm wrench, you will be able to exert much more than the 29 ft lbs and possibly damage the thread. If you were using a 2 foot/24 inch/60 cm wrench, much much more.

So I would agree with the other replies with a proviso - a torque wrench isn't required in the sense that this doesn't require exact torque. And for someone who knows roughly what 29 ft/lbs (foot-pounds, 40 Nm) is, that's fine. For someone who has no idea, and tries to tighten as much as they can, it might be necessary to avoid damage.

And as someone pointed out, these rings can be made of aluminium - therefore easy to strip/deform.