I used to (years ago) meditate anywhere between 30min to an hour at a time. sometimes in seiza. sometimes in anza.
Now I meditate for about 5 minutes or less in seiza. approx three times a week. I might just relax my breathing and slow my heart or visualize a kata/technique...or on rare occations, I don't think anything. I don't decide on something prior to sitting.
The reason for the change in amount of time I put into meditation is simply because I get the most out of it during the first 5 minutes or less. anything more and it feels like I'm forcing myself to 'do' something.
everyone is different. meditation is personal and you will find it helps you in some ways that others don't identify with. do your own thing. if a bench enables you, then use it. for me, kneeling on a bench, sitting in a chair, or using a seiza bench interfers with lowering my breathing to my abdomen.