There are a number of commercial software packages that will let you do it with only two runs (DryLab, ChromSword, ACD Chrom Simulator).
For linear gradients, you can get "in the ballpark" with a single initial scouting run from this relationship:
tG = (k* x deltaPhi x Vm x S) / F
where:
tG is the gradient time
k* is your target for average k' (5 is a good general choice here)
deltaPhi is the gradient range expressed as a decimal fraction (e.g., 5-100% would be 0.95)
Vm is your column internal volume (void volume)
F is the flow rate
S is the slope of the log (k') vs Phi plot (5 is a reasonable value for small molecules).
The background explanation is far too long for a post. For how this fits in to an overall method development strategy, check out the Snyder, Glajch, & Kirkland
Practical HPLC Method Development book, and for more detail on the theory behind that equation, check the Snyder and Dolan book on
High-Performance Gradient Elution: The Practical Application of the Linear-Solvent-Strength Model