Is this a common use: Upscaling DVDs to 4K

HI tried a couple of titles and it seems to do a decent job.
Since have bought it yet is a bit hard to tell with the large watermark.

One title takes about 12h on my computer which
is a long time.

If it is not a goo idea are there other products that would be better suited for it?

Dependend on your computing power 12 hours for a full movie is quite normal. In the early days of Video AI when using the Gaia modes it took around 18 hours. But with an 4090 card you might get the job done in two or three hours.

I guess this is a quite common use - although many will settle for DVD → FullHD upscales like me.

And yes, Topaz VideoAI does a very good job on this task, especially with the IRIS model.