As a TA, Should I allow students to ask their doubts without schedule?
From the student's standpoint it is very helpful to learning to be able to ask questions when they arise, rather than waiting hours or days for the opportunity. But if you are managing a number of students you need to be able to respond to student needs both effectively and efficiently. Also, responding to student questions doesn't always mean actually answering them. More often, actually, you can show them where to get the answers themselves. Their learning will be deeper if they put effort into it, so make them do that. One exception is that you sometimes need to dispel misconceptions that people have so that they don't waste time on unproductive study. 

One of the best techniques that I used in the past (I'm retired now) extensively was an email list to which I and all the students were subscribed. I didn't answer questions sent to my normal email - individual questions - but I always saw that student questions directed to the mailing list were answered. But other students were also encouraged to answer questions posted to the list, so that not all answers came from me. I normally had to answer about a third to a half of the questions. Moreover, every student was able to see every question and the answer. Then, you don't have to answer the same question multiple times. And you also have a record of questions that can be useful for future course preparation. 

I had to monitor the list, of course, in case incorrect answers were given, and I had to set some ground rules so that answers to exercises weren't given, but generally, the list was very open. People were encouraged to help their classmates learn, by pointing to resources, for example. 

Such a list is also helpful if some students are dissatisfied with the course and complain on the list. I never had to "defend" myself on the list but occasionally students would themselves respond to complaints of their peers. 

There are, I assume, other ways to do this, but the keys were (a) complete transparency of questions and answers, and (b) ask a question or express a concern at any time, day or night.