BUT, can someone tell me approx how long I could expect a 2.5lb tank
to last in a 10g and 20g (half as long I would guess).

I understand that it will vary depending on certain variables....I
just need a rough estimate, ie, 4-5-6 months?

my 5lb bottle, in a 29g mod. planted tank, with 1-3bubbles per sec lasts me
nearly 4 months... @$6.00 per fillup... so even if it was only 2 months, and
costed $10 per fillup, I'd still do it... best algae control you can
buy.....