Did a quick test and it looks like the data is a type 'Timeticks' which is a 32 bit integer. From the RFC:
The TimeTicks type represents a non-negative integer which represents
the time, modulo 2^32 (4294967296 decimal), in hundredths of a second
between two epochs. When objects are defined which use this ASN.1
type, the description of the object identifies both of the reference
epochs.
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[user@host ~]$ snmpwalk -v 2c -c $COMMUNITY $DEVICE sysUpTime
DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (1342832478) 155 days, 10:05:24.78