Details are EVERYTHING. Since they pinpoint both the cause and remedy.
When you say " 2TB Time Machine", what you mean? Time Machine is an application, not a device. "2TB" would imply a device (drive). Whether this is a drive hooked up directly via say, Thunderbolt or at the other end of the speed spectrum, wirelessly, can make all the diffference.
Are we to assume you really mean a 2 TB Time Capsule?
If so, are you backing up over a wireless network? If this is the case, then Time Machine's calculation is based on the current connections speed, at that moment. Check your network connection speed, and it may explain the extended time required.