when you first start working out, you will be asking your muscles to do more than they have been lately. Your nervous system will react first, recruiting more motor units to do the job - and over time you will be able to move more weight (get stronger) with your existing muscle. [It's kind of like you're asking a room full of people to volunteer to stuff envelopes - if you ask for 3 or 4 all the time, only 3 or 4 are going to volunteer. If you up the demand until the whole room has to react, you can get more envelopes done. You've recruited more motor units to accomplish the task].

Once the demand goes beyond what your existing muscle can handle - after this neural adaptation - you will start to make new muscle to accomodate the demand. Is this what you were asking? This neural adaptation can take a few weeks - so you get stronger without gaining any mass.

or were you asking how soon after a workout do you start to make new muscle? If this is the question - you make new muscle as an adaptation response - so it probably starts a few hours after you finish working out an continues for many hours - even days. But it will likely take several workouts to trigger this response - your body isn't going to make new muscle (an expensive process) unless it is *sure* the demand is really going to continue for it.