Advice from Igor:
The length of a song doesn't, for the most part, affect the CPU. However, here's a huge tip to help you out:
In CPU hungry softsynths, reduce the polyphony to the bare minimun you need to play the composed part. For instance, some of the control trip setings "boot up" with plugin polyphonys of 32, which in Sculpture, will eat up tons of CPU. The same concept applies to the EXS and other "CPU friendly" plugins too. I always "trim" my polyphony to match the part.
Also, if you have lot's of plugins on the "output" channel to "master as you go", his, too, will tax the system. In a CPU intensive song, do your mastering "after" you mix/bounce as a separate "mastering" bounce.
Here's a list of the hungriest plugins:
Sculpture- especially when you have string animation enabled, AND, even more important, when you have the ADSR's attack set above "0"... because when it's set above "0" Sculpture re-attacks an already vibrating wave and the calculations are astronomical.
Space Designer -The longer the delay, the more CPU!
Linear Phase EQ
Guitar Amp Pro -I like to use this a lot and it really eats up CPU.
And any plugin that has a "look-ahead" function eneabled.
I hope this helps!