Timelines slip for a small number of predictable reasons, and almost none of them are the engineering taking longer than expected. They are content that nobody wrote, decisions nobody made, integrations owned by other people, and app review. A schedule that does not account for those four is a wish.
How long does it take to create a website? Four things consume the schedule
Content, because it is the client's job and competes with the client's real job. Decisions, because work stops while someone with authority is unavailable. Third party integrations, because the other party has its own priorities and release calendar. And feedback rounds, because each one has latency on your side. Ask any bidder how many client decisions the schedule assumes and by when, and the plan becomes a shared commitment rather than a prediction about them.
How long does it take to make an app? Review is a hard constraint web does not have
Mobile releases go through review, and a rejection costs days at exactly the wrong moment. Apple's App Store Review Guidelines are specific about the recurring causes, including in app account deletion where account creation is offered, permission justification and subscription disclosure, and designing for them in advance is far cheaper than discovering them during a release week. Build at least one review cycle into the plan, and name who responds to feedback and in which time zone.
Ask for a web development timeline by dependency, not by phase
A phase based plan hides who is blocking whom. Ask instead for the list of things the agency needs from you and when, and the list of external systems it depends on and who owns each. That version of the plan is honest, it makes your own obligations visible before you commit, and it is the version that survives contact with reality. A supplier that resists producing it is protecting an assumption.
Judge the estimate rather than the duration
The GAO's cost estimating guide makes the general point well: what makes an estimate credible is stated assumptions, a work breakdown and a risk analysis, not confidence. Ask each bidder for the three things most likely to make its schedule wrong. A bidder that names its own risks is far more likely to land near its number than one presenting a clean plan with no uncertainty in it.
Questions people ask about how long does it take to build a website
How long does app development take, and why do projects slip?
Content nobody wrote, decisions nobody made, integrations owned by other teams, and app review. Engineering taking longer than expected is rarely the cause, which is why a schedule that does not name those four will not hold.
How long does it take to create an app, and what is different about app timelines?
Store review. A rejection costs days at the worst moment, and the recurring causes are published: in-app account deletion where accounts are offered, permission justification and subscription disclosure. Build in a review cycle and name who responds.
How should we ask for a website development timeline?
By dependency rather than by phase: what the agency needs from you and when, and which external systems it depends on and who owns each. That version makes your own obligations visible before you commit.