Dear Rabbi, My trips to bathroom are very frequent. Sometimes twice within 30 minutes and so on. I wonder if I should say Asher Yatzar every time I go to restroom. In another word, is there any time span that I have to keep in mind between saying one Asher Yatzar and another? Thank you for your time.

Shalom, You should say every time asher yatzar unless you need to go again right after you use the restroom, which in that case you can't speak torah or pray when you really need to use the restroom. See mishna Brura (7;2). All the best!