Microsoft says its year-long offer will expire on July 29, 2016. After that date, unless a new offer materializes, you will need to pay for a Windows 10 upgrade of $119 for a Home upgrade or $199 for the Pro edition. Over three hundred million users have already converted to Windows 10, so it appears to be safe to upgrade now.
If you stick with an old version, just because you are comfortable, get over it. Older versions become more and more susceptible to malware attacks, which are much worse to deal with, as well as being more expensive than just upgrading for free.