20 min to 30 min sounds very reasonable. It takes time for the cathode to heat up so that enough electrons are available for "processing". Mine takes about 25 min before I can do an image restore (and image restore requires monitor to be warmed up, though I'm not sure how the monitor detects when it's warmed up).
Uncle Vito likes to warm his monitors up for 1.5-2 hours before working on them.
According to the manual - the monitor lets you image restore after its been on for 30 mins. So Sony considers a proper warmup period time of about 30 mins. Since my image looks really good at that point I won't worry about it then.