There are no rules, but as many other have mentioned 10% of the equity is a benchmark for a co-investment. That contribution could be derived from cash, a rolled in acquisition fee, or a contribution from a member of the GP that is more of a strategic capital partner providing a co-investment and perhaps a balance sheet/guarantee.
The only thing a co-investment can tell you is how deep the pockets of at least a member the GP are and possibly how confident they are in the project if they are contributing more than the acquisition fee.
That being said a great sponsor may be marketing a great deal and may have just co-invested in their last 5 deals and could be tapped out and need the acquisition fee to pay back pursuit costs, back end expenses, and to cover future expenses of finding the next deal. A low, or no co-investment isn't necessarily a major concern but is something that needs to be explored more.
I'm saying that even though we now co-invest 10-25% of the equity in our syndicated projects.