Any ideas how long gmail cache DNS records ?

In typical "Google knows best" style they appear to be ignoring SOA and TTL and doing their own thing. Changed DNS severs and MX records, other public mail services have picked it up no problem. Gmail however appear to be insisting on continuing to deliver to the old mail servers for god knows how much longer ? Any ideas how long I can expect this to go on for before they Do The Right Thing (TM) ?