I had never heard of a maximum, interesting question. Considering businesses with gross sales less than 50 Million are considered Small Business, most of us here are probably never going to come near any kind of maximum expenses allowed. Unless they base it on a percentage of gross sales, then it could happen I guess. Don't know where to look except to ask your accountant.
My understanding is that you need to show a "profit motive". How that is defined and demonstrated seems to be up to interpretation (of th IRS agent, naturally). I've never heard of a maximum expenditure limit. I'd say as long as you are making sound purchases and trying to get business, you're ok. Now if you never advertised, turned down business unreasonably, consistently bid purposely high and bought three new 4500s, two wheel loaders and a tri axle truck for one driver, your profit motive might be questionable.
Two types of accounting, accruel (accounting for revenues and expenses as they occur) and cash basis (accounting when you receive or spend money). Most of us report on a cash basis for taxes. You should keep your books on the accruel basis for management purposes. When you make the conversion at tax time from the accruel to the cash basis, sometimes you'll see and gain or loss that report for taxes that didn't exist on an accruel basis. This can be tricky when tax planning. Hence another reason an accountant is helpful...
You can take a loss all day long if you want, having more expenses than revenue. However, you can't then use sec 179 to write off equipment. You must show profit to use sec 179, and you can't use sec 179 to create a loss. Year's past the limit was I think $17,500, I think this year it's $25,000. So if you bought $20 K in equipment, and have a $15 K profit, you'll only be able write off the $15,000 and then you'll have to depreciate the remaining $5,000.
But for regular operating expenses, there is no limit to how large of a loss you can take. Then again, you can only truly do this for so long before you go bust.