the standard trend for codex release in the past has been quite random with but GW have bucked the trend with 2013 so far
tau (confirmed in april)
thats nearly 1 a month with new models and the rumour mill now says marines will be out in june. Now in the olden days they could never do this as each codex was better than the last to force players to buy more armies but their hasnt been a codex creap 6th ed has rebooted everything an codex's are updates to bring the rules in line with the new core rules rather than abuse them to make more money. i think this new rapid fire release is very gd for the game its quickly closing the gap.
this year is very exciting for 40k and well hope fantasy does the same