Make sure that the contract specifies the length of time allowed for approvals or decisions.
When drawings or documents are sent to the client for approval, you need to know (and specify) when they will be approved.
Allow for this time in your plan, and make sure the client knows this is in the plan.
Be sure that the contract specifies what will happen if approvals are delayed beyond the specified time (extra payments, extensions of time, etc).
What do you think is the ideal time allocated for the approval in small to mid-sized projects?
That depends on what industry the project is in. My experience is with construction in the water industry and in engineering design in civil infrastructure.
For drawings and designs in the water industry I allowed two weeks for the client to review and respond (and hopefully approve). I would ask for responses within one week if they wanted major changes, with the aim still to have final approval within two weeks.
For larger projects (rail, roads, bridges) we go through a very thorough process of initial design checks, team checks (others disciplines seeing if the work affects theirs, and finally submittal to the client for approval. Each stage is given two weeks, and by the time it gets to the client it should not have errors or design mistakes, it should be for them to check that it doesn’t affect their other plans.
I believe that for good design, you should not be asking the client to approve unless you are confident the client would approve it (so there should be no reason they would reject it based on design flaws or your mistakes).
IT projects or procurement projects may have completely different timelines for approval. Someone with experience in those areas may need to comment on those.