This depends on many factors that I don't know, so I can't give you a definite answer:
Are you building a single balloon "String-of-Pearl" arch with helium filled balloons?
Are you creating a clustered balloon arch with air or helium filled balloons?
What device do you use to inflate your balloons (helium tank, electric pump, manual pump, by mouth)?
How experienced are you with balloon decorating? Have you ever done it before?
Professionals say that they can inflate and tie about 100 balloons (11" latex) in 20min on their own, or in 12min together with a partner.
Assuming that you want to build a balloon arch with 11" balloon clusters and that your arch would be wider than tall, you would have an arch length of 16 feet, according to our arch length formula. For a 4 cluster balloon arch you need about 6 x 11" balloons per foot. 6 balloons x 16 feet = 96 balloons in 24 clusters.
If you were a pro, you could create this balloon arch in about 30min. If you are a beginner, I would calculate at least 2 hours and one or two helpers. If you have more people you might spend more time giving instructions than building your arch.
