Well, I'm working on a how-to book myself. I feel that it's appropriate to make it as long as it needs to be to provide an adequate answer the questions your target audience is likely to ask. I would not set a maximum length on a how-to book, as this is one area where a thousand page tome from a first time author might not scare away readers.
On the other hand, it's probably a good idea to set a minimum length. If the book is mostly words, I'd aim for novel length. My book is currently about 80,000 words and is likely to grow somewhat as I revise it. One that relies on illustrations, however, may have considerably fewer words. I suspect some 150 page how-to books like the ones in Home Depot may be less than 20,000 words, but I haven't tried to verify this.