I do not have a meter to measure my co2. My whole grow is done on the cheap. However, I have about 20 years of experience with using dryice.
There is a true story floating around about a burgular, trying to escape being pursued by the police, and he runs past a parked ice cream truck, and pops open the freezer to hide inside. Well, they found him the next morning when the owner came out to start and check on his truck, the burgular died from the dryice he was using to freeze the ice cream.
Dry ice can be dangerous if you don't respect it.
Dry ice produces 8.7 cubic feet of gas per pound of dry ice.
I was talking with a friend, and explained if you want to control the evaporation, you can place the dryice into a styrofoam ice chest, heat a screwdriver, and make a hole in the chest to allow the co2 to excape onto your babies.
For my purposes, I want it mostly exposed for cooling purposes. It certainly does produce some wild looking buds.
So far as how long it'll last, 50lbs (exposed) will last me about 3 days, double that if I use an ice chest. I mostly use it exposed though.
Price is a concern, as most re-sellers aren't too happy to give out wholesale pricing. They want to charge as much as possible, I've seen places charging as much as $1.50 per pound. You gotta realize that they are always working against the clock with dryice.
I am in an industry that uses alot of dryice, so I can get it wholesale very easily, and I pay about $14.00 per 50lb block.
You may think at first that even $14.00 for 3 days worth of co2 is expensive, but it really isnt when you consider it is 3 days of constant co2 with no venting needed at all, except, I do exhaust the room at the end of each light cycle.
The effects of using dryice are readily visible on the plants too. Even tanks in my opinion, dont compare to dryice.
My last grow was done using mainly dryice, and only 2 other people knew I grow, my wife and my best friend, and each and every day, the three of us would go running in there to see the difference in growth and all the resin coated buds & leaves, this grow was done using mainly a tank, and there is no comparison. This grow is boring in comparison.
So far as using the ice chest is concerned, you could expect it to last atleast 8 days this way.
Again, there are always other ways to grow, but at $14 bucks for a 50 lb block of dryice, and about $6 bucks for a sryrofoam chest, thats not too bad for 8 days of co2, if you need to do it on the cheap.
I do have, and use tanks though, it just dosent seem to compare to the benefits of dryice.
Peace