#105 minimum period of drying for RCC roofs within the support by wooden plank and rods.

These bamboos and other supports are called propping. These are given in order to support the slab until the set concrete gains it's intended compressive strength. So, ideally, the supports should be placed for a minimum duration of 28 days, that is until the time the concrete achieves it's 100% or higher strength. 

Now, coming to your question about the contractors who are not following this practice, actually concrete achieves 2/3rd or 67% of its compressive strength in 7 days, so if the grade of concrete is M40 then it would reach a strength of 26-27 n/mm^2 in 7 days which is about enough for the slab. 
So, to sum up, although the practice followed by the contractor is not advised at all, but it is not the end of the world if they don't follow.

According to the latest IS code, the IS 456:2000, following is the list  of minimum number of days required for stripping of shuttering:

(a) Slabs (props left under): 3 days 
(b) Beam bottoms (props left under): 7 days 
(c) Removal of props under slabs: 
 (i)Spanning less than 4.5 m - 7 days;
(ii) Spanning up to 4.5 m - 14 days

Looks like the span of the building in the photograph is above 4.5 m. So make sure the shuttering stays there for 3 days and props for 14 days.

Actually concrete attains 65% of strength in 7 days of curing and reaches nearly 75 to 80% in 14 days. Hence the centring can be removed after 14 days. But it should not be subjected to load like piling of bricks, heap of sand etc till 28 days. Construction can be carried out after 14 days of curing. But pl note that keep the surface of the slab atleast wet always upto 28 days. 

Thanks and Regards
RCC Construction (Rialto Core Civil, Pratapnagar)