the only time limited aspect of the project was picking it up - you have done that. you can now take all the time you like to complete it.
however the longer you take to complete it, the greater the risk that you may miss the chance to start the next time limited project (which may or may not be an issue if it is not interesting to your fleet)
when a special project becomes available, it will only be available for a limited time. You must start the project before the end of the feature. Once you?ve started the project, though, you can keep working on it until it?s finished, even if the special event ends ? but remember, you can only work on one special project at a time, so you?ll want to finish quickly to make sure that you can work on the next special feature!
What he said: Once you start a project, it doesn't end until you finish it.
"My frozen dairy-based confectionery attracts all the males of the species to the facilities. They all agree on it's superiority. Indeed, it is superior to yours. I could teach you the finer details but that would require monetary recompense on your part." -The Milkshake Song: Vulcan Edition