hi yaya
Calum has only rolled a 5 times since he was 5.5months, he doesn't seem that interested.
Calum started gradually by first (only back to front);
lying on his back, then twisting his upper body and head to look behind him
then ages after he started to included his legs and try and get on his side - this lasted for ages.
The suddenly, he put it all together - the first couple of time he did it so fast I missed it, then I watched him again, so I practise it with him.
By the sounds of your post it won't be long, but I wouldn't worry about it. The 1st time Ds did it I missed it, becasue I had given up on him doing it and was out the room.