What is the best SOA EXPIRE number

The best is subjective:

RFC1912 suggests 2-4 weeks. This is how long a secondary/slave nameserver will wait before considering its DNS data stale if it can't reach the primary name server.

we normally use 2419200 which is 4 weeks..

intodns.com (I love them) is an automatic service. It reports anything uncommon, which may or may not represent a real problem.

As for expire specifically, very high values rarely affect anything, and in some setups SOA expire is not relevant at all.

As for RFC 1912, it's suggestion about expire is just that - suggestion if one wants to play no-brainer he can just use the suggestion. If one knows how his DNS infrastructure works, he can decide himself, and any value he choose will not be violation of the RFC.