This depends on the OS (Windows, Mac, Linux), the filesystem (NTFS, FAT32, Mac OS Extended, ext3), the size of your hard drive, the free space on the hard drive, and the laptop’s usual purpose.
If your laptop runs Mac or Linux, you will never have to defragment your hard drive. This is only useful on Windows NTFS and FAT32 filesystems. In addition, if your drive has over 50% free space, I wouldn’t worry too much about it. You shouldn’t see a performance gain unless you’re low in free space in the first place.
The computer’s usual application is important as well. If you use it for web browsing 95% of the time, it’ll be a waste of time. Gaming, maybe. Video editing, definitely.