I don't think ECO has anything to do with how soon heat is available. ECO is suppose to conserve energy. Near as I can figure it (at least for heat), the only thing it does is reduce fan speed a couple of bars. When I'm in auto and heating, I hit the ECO button and the fan will drop down a couple of bars, that's about all I notice.

Two climate control systems, one inside and the other at the tailpipe.

2007 Camry Hybrid (in service June 2006)
FE: 6.77L/100km (34.8 mpg) over 37750km (23450mi)
75% city - Ave trip 20 min - 2 Winters