TTL question

[mahermick at gmail.com]

Hi Folks,

I'm trying to understand the implications of not lowering the TTL for a
record prior to it being changed.

Taking a hypothetical situation - I have an Primary MX record with TTL
of 3600 (1 hour) and no Secondarys (for arguements sake).

If I changed the MX record would the maximum time my mail would be
unavailable be one hour or is it possible that it could take longer?

Thanks in advance,

Mick