If he wanted to enter her dream, he would have to wait until she was fast asleep. But when would that be? The alchemy process was long and tiring, and not to mention very physically draining. She would definitely take a rest when she was done, right?
It would be better if he could watch her first before entering her dream as opposed to directly entering her dream, where he could not tell what was happening around her. The skill of entering other people's dreams required a great amount of spiritual power to perform, so he would not be able to use it repeatedly.
Desperately, Di Fuyi tried to use his spiritual power to put his burning desire to an end while calculating the time he had left. He thought that she would be asleep very soon, so he started to perform the skill right away.