No hay diferencia. The meaning is the same. "how long" is more commonly spoken. "how much time" may be asked when a specific answer is required for setting a timer.

how long -- more or less pasado indef IMPLIED unless you add how long WILL IT TAKE/BE, then it is futuro indicativo how much time - more or less subj futuro , but you can make it more subj futuro with how much time WILL IT TAKE, or you could convert it to past indef if you said how much time did it take ( pero how long es mas comun)

"How long" indica una medida y se puede usar en varias situaciones, mientras "how much time" es una medida del tiempo. Cualquiera de las dos se puede usar pero el uso de "how long" es mas comun.