how many hours to replace spark plugs on 2007 sienna van

More like minutes, 30 at the most for a shop ready to work.

Any shop ready to work can replace the front 3 in about 1/2 that time. The rear 3 plugs require removal of upper intake components and can take up to about 3-4 hours regardless of preparedness to work. If you would like to see why and maybe tackle the job yourself, here is a decent article showing why 30 minutes is laughable. One large error in the video is the mechanic in the video names the exhaust manifold as needing removal, that is actually an intake manifold, plastic would melt exposed to combustion gas temperatures. The procedure is correct even if article itself needs correction. And I recommend NGK iridium plugs for Toyotas. https://axleaddict.com/auto-repair/How-to- change-the-spark-plugs-in-your-2004-2005- 2006-2007-2008-Toyota-Sienna-step-by-step