There is no one flower which can be guaranteed to have the longest blooming time, as this depends hugely on the individual environment. You can take some seeds from the same plant, for example, and plant them in different places - even different parts of the same garden - and their growth and bloom will still vary. There are just too many factors like sun, shade, temperature, rainfall etc, as well as the fact that the plants themselves aren't identical.

However, if you type a phrase like 'long blooming flowers' in a search engine, you can find plenty of helpful information, often from individual gardeners. If you click here you can find a list of long blooming perennials, as well as information on how long each flower can be expected to last and how best to care for them.

You could also join a forum like Gardening Questions, which can tell you where to find further information.