12 hours.
.... max, (minus the DNSs that substitute their own longer TTLs for authoritative TTLs they think are too short.)
On average, TTL/2.
In practice, traffic begins hitting the new IPs within minutes, if not sooner.
Finally, no need to lower the TTL on the MX record (unless you are changing RDATA field), lower only the TTL on the current MX hostname's A record, which is where the new IP will be published.
There is no problem in lowering, at <current TTL> before the change, the TTL of the A record to 5 mins.
<Old TTL> after the change, raise the A record's TTL to your usual value.
Len
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