You will not be able to hit it very accurately on your first attempt - it takes considerable experience at this sort of activity to be very accurate in estimating. There are industry "standards" for this - the Landscape Data Manual published by the California Landscape Contractors Association breaks it down like this: for a crew of one (you), they estimate 12 minutes for planting one 1 gallon container, 30 minutes for one 5 gallon container and 40 minutes for a 10 gallon container. Anthing much larger than that generally requires more than a single individual. And it does not factor in B&B material, which is typically large and needs big planting holes and is heavy. This is assuming the soil has been properly prepared/amended and plants placed but planting holes not yet dug. And you will need to factor in time for clean up, spreading mulch, watering, etc. Depending on the number of plants to be planted and your stamina, the later in the day, the slower you will get :-) And remember that the more carefully handled and planted the material, the better the long term results will be, which I'm sure your friend will appreciate.

My best advice is estimate on the generous side, time-wise, with the option of reducing the estimate at completion. Typically when doing work for friends, family or acquaintances, we have a tendency to want to cut them a "deal" and that will often come back to bite you when getting paid for all that hard work :-) Either make it a very professional arrangement or be a friend - it's difficult to do both.