Before you decide on a provider, you can ask them for a quote and how long the entire process will take. The duration depends on the complexity and size of your solar panel system.
It takes a few weeks to get quotes and home visits from installers. But once you decide on a provider, the process will not take much longer.
As soon as the scaffolding is complete, the actual installation should be done within a day in the case of regular sized panels. However, bigger panels might take a more prolonged period.