I did do some longer sessions (30-60m) in the beginning, which may have helped it to "click", but -- YMMV -- for me, ~10m is the threshold where it can suffice for the day. Increasingly in recent months I'm able to effectively meditate while doing other things, and it's more a deliberate letting go and and complete immersion in the present than a focus of attention on breathing... but starting my day w zazen is essential.