Potatoes nearly reaching top of grobag.....!

I have in my greenhouse 3 potato bags, I would say they are about 50cm tall when fully rolled up.  I have charlottes in (not even broken the surface of initial compost),  and one of the other one's (premiere or Rocket - can't remember which) is growing steadily, but the third one ir romping away. 

It said on instructions when 15cm of growth had occured, put 10cm of compost on it, so I did so and have been putting compost on it most days (leaving some showing).  I now only have about 3" left before reaching the top of the bag. At this pace it will be by Monday - what do I do then?

When are they likely to be ready? I pressume this is when the foliage starts to die?

I will move them outside after the risk of frost has passed.  I might try moving them outside in the day if I can (a bit 'eavy) and back in at night

this is my first year with bags as well but I would imagine the usual rules apply,
ie, if they are earlies, wait untill they flower and then scrape back the soil to see if there are any big enough to harvest.
for mains wait untill they die back

Could be wrong though