I was a member of the Virginia Militia in Colonial Williamsburg for many years. And in that role, fired many times each day an 18th Century musket (the loading procedures though were identical to earlier MUSKETS).
"Casual" loading, from a previous firing to being ready to pull the trigger for the next fire takes about 15 seconds.
However, in close quarter rank volley mode (one line fires as the others reload - as quickly as possible) most could load in as fast as 5-8 seconds, easily supporting a three line, 10 second separation volley - including back line moving forward. But in this rank volley mode, probably about 2 out of 10 either wouldn't make it in time or have a misfire due to (usually) bad priming. In casual mode, 15 seconds is more than enough time for a successful firing.
--- Addendum ---
What I gave above are LOADING times. This is different from firing separations. Because muskets are horrendously inaccurate, volley lines were a must. So firing separations (typically 20 seconds) also include firing commands being issued. Also, the times (and those for other answers) are based on troops equipped with load pouches. You had a small cloth bag with a premeasured amount of gun power. You would dump the contents of the bag into the musket then use the bag itself as the wadding. Especially in the 17th century, use of power horns and separate wadding was standard which significantly increased time to load.