Onshore wind has been happening for decades. See for example the 1.5GW farm in California, the 1GW farm in New Mexico, the 1GW one in Oklahoma, the 900MW one in Texas, or the 845MW one in Oregon.
Offshore has a rather fast construction time, it turns out. For example, the United Kingdom's Hornsea Wind Farm Project 2 was given planning permission in 2016, and it reached its full capacity of 1.4GW less than six years later. Project 1 at the same wind farm reached 1.2GW in less than five years.
And when it comes to cost, Hornsea Project 3 is to start construction next year - with commercial operation scheduled in 2025 - at $12bn for 2.4GW. Not bad when you compare it to Finland's Olkiluoto Nuclear Power Plant unit 3 costing an estimated $11bn for 1.6GW - which took 22 years from first license application to design output power.