Your countertop options

As you know both Home Depot and Lowes carry their stock countertops in 4, 6, 8 and 10 foot lenghts. I'm assuming that you want to reduce the seams in your kitchen. You're not going to be able to find that 12 foot length in stock. If it is any consolation, people who install granite have the same issue. There are seams in the granite. Tight ones though.

I'm not sure of your layout or where your kitchen sink is going but it seems that you're going to have to break the 12 foot length into small sizes. You can do two 6 foot lenghts or an 8 and 4 foot pieces. If your budget is tight, this is the way to go. The in stock countertops are easy to work with and you don't have to deal with separate back splashes either. We actually cut the countertops and can assure you that if you have a table saw and a very fine tooth saw blade, it will come out fine.

I would not even consider concrete unless your budget is substantially more than the $140 that the two 6 foot long countertops will cost you. Feel free to provide more information if I have missed the mark here and welcome to the community.