There are no 64bit counters for this value available. The maximum value this counter can have are 2147483647.
That means the maximum shown time would be 248 days.
If this device only shows 50 days then it is a firmware bug on the device.
From the definition of the MIB
TimeInterval
A period of time, measured in units of 0.01 seconds.
TEXTUAL-CONVENTION
INTEGER 0..2147483647
cipSecTunActiveTime 1.3.6.1.4.1.9.9.171.1.3.2.1.10
The length of time the IPsec Phase-2 Tunnel has been active in hundredths of seconds.
Status: current Access: read-only
OBJECT-TYPE
TimeInterval