Only one evolve can be carried out at a time and each evolve takes approximately 30 seconds. Thus, to make maximum use of a Lucky Egg you need to run an evolution blitz that stacks 60 evolves to run sequentially in 30 minutes. This sequence will produce a 60,000 point experience gain.

How many evolutions can you do in 30 minutes?

One single evolution takes exactly(ish) 25 seconds for the animation to fully play out, meaning that you can't do anything during that time. So if we account for user speed we can estimate that you can do 2 a minute, meaning you would be able to do 60 evolution's in a 30 minute period.

Should I use a lucky egg when evolving?

If you use your Lucky Eggs during this time, you'd be able to get a massive 4x XP. With 2000 XP per evolved Pidgey and 4000 XP for a hatched 10km, it's absolutely worth waiting if possible.