Another PET bottle question

[gullarm]

I filled my 2 litre PET Bottles last night

I used 100 grams of sugar boiled in water to ensure it was fully mixed and clean, then force cooled.

I then pitched this into the beer, stirred and left for 20 mins then stirred again before bottling.

Now I can see the yeast is dropping to the bottom of the bottles but they havent gone as firm as I hoped, nothing like the firmness of a normal PET bottle under pressure.

They are sitting in my kitchen and was hoping to move to my shed to lager for a good few weeks.

How long do I have to wait, or is it better to use coopers bottling drops.

[GTOrichie]

I leave them in a warm place for a few days to build up the gas then move to a cold area for a few weeks before sampling the first bottle