I square log cabin blocks (and log cabin variations) each time I complete 4 rows around. My largest square ruler is 22". I don't use it often, but it's nice to have when I need it.
On other block patterns, I check the size each time I add a large or intricate piece. It's the only way to make sure your block is going to be the right size when it's finished. If you wait until your block is finished to square it, you have to take the block completely apart and redo it if the size is incorrect.