This study by Parsons Brinckerhoff considers the achievable start-up times that could be applied to different types of fossil fuel power generation technologies, in order to provide electricity to the national grid. The study also looks at how long it takes to mothball and de-mothball those fossil fuel power generation technologies

https://www.gov.uk/government/publications/technical-assessment-of-the-operation-of-coal-gas-fired-plants