GTA 3 is an incredibly large game, and the amount of time it takes to finish will vary significantly depending on a few factors, such as how much time you have to dedicate to playing the game, how many side missions and objectives you choose to pursue, and how quickly you complete those missions.
If you are focused on completing the main story missions as quickly as possible, you can expect the game to take anywhere between eight and twelve hours to complete. However, if you choose to pursue all of the side missions, complete the various challenges, and collect all of the hidden items, you can expect the game to take significantly longer.
For most players, the completion of GTA 3 can take between twenty and thirty hours. This may sound like a long time to complete a game, but it is important to remember that GTA 3 is an incredibly expansive and detailed game with plenty of content to explore. There is no wrong way to play the game, and many players will find that the journey of completing GTA 3 is as enjoyable as the end result.