The other answers I've seen so far offer some general insights. Here, I want to share some sources that go deeper. They discuss equations which help better understand the quantitative relationships between power, speed, air resistance, drafting, and so on.
To get warmed up, check out this interesting but accessible article by Marilyn Trout: Relationship Between Drafting and Climbing.
Trout quotes Cycling Uphill and Downhill by David Swain at length. For example: "At very slow speeds (on the order of 16 km/hr or less) air resistance is negligible, and drafting becomes nearly meaningless."
But why is the "magic" number 16 km/hr? Let me say: there is no single magic threshold. To figure out that threshold, you have to define, first, what percentage is "small enough" for your question at hand. For example, if you were to ask, at what point does air resistance stop contributing more than 0.5% of your total power output, an equation can give you an answer.
At the elite levels, I would make a rough guess that as little as 1 Watt over the course of a long climb could make the difference between victory and second place. My point is this: don't assume drafting is negligible until you have done your homework on what "negligible" means.
And how do you do your homework? Asking here for thoughts on hill-climbing and drafting is one place to start. But if you want to get a better understanding, ask for references and then read scientific papers. Inside you will find studies and equations.
Trout mentions this equation from Swain, who cites its source as Equation of motion of a cyclist by P. E. di Prampero, G. Cortili, P. Mognoni, and F. Saibene.
W = (kr M s) + (ka A s v^2) + (g i M s)
- W is power
- kr is the rolling resistance coefficient
- M is the combined mass of cyclist and bicycle
- s is the bicycle speed on the road
- ka is the air resistance coefficient
- A is the combined frontal area of cyclist and bicycle
- v is the bicycle speed through the air (i.e. road speed plus head wind speed)
- g is the gravitational acceleration constant
- i is the road incline (grade; however, this is only an approximation, as the sine of the road angle to the horizontal should technically be used)
If you want to dig into the aerodynamic benefits of drafting, I would recommend checking out The understanding and development of cycling
aerodynamics by Lukes, Chin, and Haake. In particular, check out the section on drafting on page 67.
Drafting behind a single rider with a 0.2 to 0.5 m gap was found to reduce oxygen consumption by 18 ± 11% at 32 km/h and 27 ± 8% at 37 km/h and
Drafting behind one, two and four riders resulted in the same oxygen consumption reduction at 40 km/h (27 ± 7%).