Loft constructions projects tend to take at least a year, possibly two. It does seem to be dragging on though, partly I assume because of legal and other administrative reasons.
As a rule of thumb, properties should appreciate by 20% after a high quality building renovation (new lift, etc.) in a good location. There are various considerations though, e.g. if the renovation affected the light your flat gets, or if it somehow has a negative effect on your security/position in the building, etc.
If you need the cash, then it would seem to make sense to sell up and move on now. Nobody is really predicting an upswing anytime within the next 2-3 years. After that, it's still a guess, but long-term prospects appear to be good. Rentability shouldn't be a major issue if your apartment is nice.