I am new in this forum.And I have a question ?
What suitable time to cross pollination begonia ?
Any time that the female flowers open - provided you have previously collected pollen or male flowers are open as well.
I am not see pollen from male flower.
I doubt you will see the pollen itself (very tiny) but you should see the open male flowers. It also depends on the begonia itself - some hybrids are sterile or the male flower may not open at all but all species will be fertile.
Here are a couple of web sites that may help.
Growing from Seed
Male and Female flowers on B. dregei