Can anyone tell me using Appian if I have to develop 30 simple forms/screens and

I'm not sure this is something anyone on here can answer easily. The amount of time required will depend on the experience of the developer(s) and the complexity of the requirements; your definition of simple or complex may vary considerably to that of others.

Might I suggest a quick PoC on a single screen to baseline the time required? Maybe then you'll have a better idea of how long the wider requirements will take.