Why is there a time limit on Tobi's dematerialization technique? This question contains spoilers for early manga readers and anime viewers

Konan mentions during her fight with Tobi that Tobi's dematerialization technique can last for only 5 minutes. This can be confirmed as true since Tobi resorts to Izanagi to escape her ultimate technique, Kami no Shisha no Jutsu. It is later revealed that Tobi's dematerialization technique is actually just teleporting himself into his alternate dimension with Kamui.

However, earlier during the Five Kage meeting arc, he had teleported Sasuke and Karin and kept them in the alternate dimension for quite a long time. He also kept Fu and Torune in the alternate dimension for several days. This shows that he does not face any significant side-effects, like draining chakra, by keeping people in the other dimension. Also, staying in the alternate dimension did not seem to have any adverse effect, on at least Sasuke and Karin.

Why can Tobi not keep himself in the alternate dimension for more than 5 minutes? In particular, why could he not just escape Konan's ultimate technique by staying in his alternate dimension for 10 minutes, rather than use Izanagi?
There's no definite answer yet.

The technique is about sending parts of yourself to the other dimension, to avoid being hit. 

Two reasons I can think of are: 


Since it's an activated mode type technique, it constantly drains his chakra. Five minutes are his limit. 
He cannot keep parts of his body separated for so long, because there could be some sort of biological limitation of transferring oxygen and blood to the organs in the alternate dimension. 


The first sounds more feasible to me, but since it isn't explicitly specified, we can't give you a definite answer.