The install is easy, its the tuning that you'd want help with. Course my experiences are at altitude and apperently that fricks with the MAP on the OER SUBCON piggyback, so tuning might be alot easier at sealevel.
I guess it all really depends on whose kit we're talking about. I know you've got a bundle of oem intake manifolds so you're probably familiar with what it takes to remove the original parts. If the kit is complete it should just require bolting on the new hardware, checking linkage adjustment and syncronizing them. I agree with previous posters that tuning is the tricky part and it really depends on what electronics are used and how familiar you are with them. You could spend as little as 1 hour getting them to run and idle but you could spend way more than that getting them perfect for all conditions. As an insurance policy you might want to build a leak down and compression test into the install price and give the owner a report of this data. This might help alleviate potential liability. Aftermarket engine management is for advanced users only and one small mistake when plugged in with the laptop computer could have catastrophic results. Whatever route you choose to go you should make it clear to customers what is included in the price, what to expect, and what additional items may be necessary such as maintenance that should be performed or dyno tuning. It will be slow going the first few installs but once you're familiar with the one specific kit you should be able to work faster and improve your profit margin.