Sorry, we only used a trailer (with great success, I should add, he slept like... well, a baby). But as I recollect the general advice was: don't put your baby in a seat until she can sit by herself, and don't have her there longer than she could sit by herself otherwise. Suspension is also an issue. Babies have big huge heavy heads and weak necks to hold it all up with. So I think realistically you're looking at less than an hour at a time, at gentle speed on non-bumpy roads. Trailers are better for babies, at least the ones that are large enough to put a reclining seat in.

They do get bigger and stronger very fast, though, so I wouldn't despair if this sounds like very little. A small child, 1-3 years old, can handle bike rides a lot better.

How about hiking instead? You can get baby carriers than can be used for very small babies that support their heads well. Worth considering for those first months?