How long must I train to see some real hypertrophy?

The way I see it, you have two good options: one, create two separate straps, one for each ring. Or two, with your current setup, you can hang a stick, wood, metal, whatever, with either an eyebolt, a shackle, or some other way to attach the ring straps to each end, so it can act as a spreader. The advantage of the second method is that you won't be boxed into your doorway, which is the nicest thing about your current setup.

I just saw a third: drape the ring straps over the two neutral grip handles.

On rope is easier then on cabel. More elasticivty of rope.

thanks for all the help! i did like you said and its now way harder to do iron cross.

Sweet Now you can get even stronger!