They don't take all that long really.. an hour perhaps. I'll be happy to help you build one, but best to do it right away after a reset, so we get the most use out of it. They are nice for blaze rods
as for your spawning problem, the blazes spawn in a 8x8 area around the cage. If you are using a minimap program, the North and West sides are 4 length, and the South and East are 3 length (away from spawner cage) I prefer to fill in the area with netherrack or dirt, to stop them spawning, and plus it's very easy to remove. Bricks or cobble are hard to remove when yer done.
I've built mebbe 10 of these now across different servers
Z