I think the difficulty resides more than anything in the amount of resources the contractor is thinking to employ to excute the works. It is more or less attainable to know (to some extent) the produce of the typical teams or machines used for the different construction units. Persist always however the uncertainties of the final dificulties in worrying soils and the amounts of means tasked. So it is not infrequent, specially when time duration is primed in the contract, to find a contractor (or group of them) offering to finish (or if already contracted, finishing) the works in nearly half the time the allowed duration, or "what one would have thought a reasonable period".
This makes difficult that whatever your guess one can be extremely precise about the duration when soil, means, budget, primes and everything that makes the timeframe be affected has significant say. If money is available as thought, and no uncertainties were extant, it would be only to sum the compatible time use of a number of means assumed in the project. But that still would be assumption.
Seen otherwise, perhaps priming is one interesting way of approximating the timeframe to some wanted; the contractor would have incentive to allocate means enough to approach the timeframe, and conversely, the more suitable contractors to the work would be more interested in it because knowing their advantage, they can think on the incentive both as a partial assurance to get the contract and as well, of keeping some minimum benefit, and so would make the bidding process more selective from a target date viewpoint.