The lack of acks is why most companies receiving money through ACH will "season" the transactions for a set amount of time. Most of the returns occur in the first 3 business days so most seasoning periods are just around that long. The reality is you can receive a reject up until (I believe) 60 days after the transaction date. I've seen a lot of companies be a little smarter here and reduce the seasoning times for repeat users using the same source bank account. It is a fascinating system to work with every day. Wire transfers are their own bundles of fun too.
Now I finally know why my credit-union (which serves the high-technology industry exclusively) told me that they couldn't put a hold or otherwise segregate any cheques I deposit until they've cleared... because they have no way of knowing when/if they've cleared!
It's 60 days, + some possibility of up to 2 extra days at the end for Fed Reserve holidays, and then in cases of outright fraud, occasionally banks will ignore the rules and return stuff later than that.
