There would not be any shar'i time limit to perform a ghusl, unless it is an obligatory ghusl and you have a short time-frame for you to complete your ghusl and perform your wajib prayer on time and before its time finsishes.
For example, you have not prayed Salat Fajr, and you must do ghusl janabah, and in 15 minutes it will be sun rise. In this case, you must do your ghusl as quick as possible, and have enough time to pray Salat Fajr within the required time.
In other cases, although you do not need to finish showering quickly, it's always important to be considerate of wasting water, as excessive use of water could also become haram. Be reasonable in how long it takes you to shower/do ghusl, and the less water you use the better. This is the Sunnah of Islam as well.
And Allah knows best.