If the session key negotiated at connection startup is used too much or for too long, it may become feasible to mount attacks against the SSH connection. Therefore, the SSH-2 protocol specifies that a new key exchange should take place every so often; this can be initiated by either the client or the server.
While this renegotiation is taking place, no data can pass through the SSH connection, so it may appear to ‘freeze’. (The occurrence of repeat key exchange is noted in the Event Log; see section 3.1.3.1.) Usually the same algorithm is used as at the start of the connection, with a similar overhead.
These options control how often PuTTY will initiate a repeat key exchange (‘rekey’). You can also force a key exchange at any time from the Special Commands menu (see section 3.1.3.2).
You might have a need to disable time-based rekeys completely for the same reasons that keepalives aren't always helpful. If you anticipate suffering a network dropout of several hours in the middle of an SSH connection, but were not actually planning to send data down that connection during those hours, then an attempted rekey in the middle of the dropout will probably cause the connection to be abandoned, whereas if rekeys are disabled then the connection should in principle survive (in the absence of interfering firewalls). See section 4.13.1 for more discussion of these issues; for these purposes, rekeys have much the same properties as keepalives. (Except that rekeys have cryptographic value in themselves, so you should bear that in mind when deciding whether to turn them off.) Note, however, the the SSH server can still initiate rekeys.
As well as specifying a value in bytes, the following shorthand can be used:
1k’ specifies 1 kilobyte (1024 bytes).
1M’ specifies 1 megabyte (1024 kilobytes).
1G’ specifies 1 gigabyte (1024 megabytes).
Disabling data-based rekeys entirely is a bad idea. The integrity, and to a lesser extent, confidentiality of the SSH-2 protocol depend in part on rekeys occuring before a 32-bit packet sequence number wraps around. Unlike time-based rekeys, data-based rekeys won't occur when the SSH connection is idle, so they shouldn't cause the same problems. The SSH-1 protocol, incidentally, has even weaker integrity protection than SSH-2 without rekeys.