Personally I have never upgraded myself via bid or the offer on the LH site. As a SEN I do get the free seat next to me in 90% of the cases and the food is not worth it.
I do it regularly for my wife + family when they are flying alone, and my rule of thumb is not to spend more than 1 - 1,25 EUR per flight minute depending on the route.
Just an example, today my wife is on FRA-ZAG and the offer was 65 EUR which was acceptable to me and since she has no status and the upgrade grants her lounge + priority services, I think it is worth.