They tend to grow approximately 1-2" a year.
Most likely from being kept in too small a tank, IMO...
June 2012, 6cm:
July 2013, 20cm:
That was in a 400L tank with a 140x65cm footprint, at 20cm+ the growth has slowed down a little despite being in a much bigger tank, but it has easily put on another 6-8cm in the past 12 months.
I doubt any large Panaque will physically outgrow a small tank in the way an arowana or even a common plec would, but I believe smaller tanks will have detrimental effects on the long-term growth and ultimate size of the fish. I'd be happy to be proven wrong, but I would put money on it that your little L190 won't grow 5" in 13 months while kept in a 29g tank, despite it looking like he has plenty of space.
BTW, the Panaque in the pics above wasn't a freak, I had three others of similar size who also put on about 4" in 12 months, and have continued to grow at a similar rate in the 2000L tank. If your juvenile royal is only growing at 1-2" per year you're doing something wrong, IMO.
That said, it's a nice looking little fish you've got there, just appreciate that it should grow into a monster, and they do make a hell of a mess when they get bigger. As for the shyness, I made the switch to only feeding mine during the day about 12 months ago and I see them a lot more often now. Granted I do have 14 plecs in the tank and a heap of other bottom feeders so I can feed heavily, but once it has settled in and is eating try offering food in the morning before the tank lights are turned on to try encourage it out.