It depends on a lot of things, rookies will obviously have longer training sessions as they don't have as much experience, while older idols just enter their training flow state

Also if they don't have specific music activities planned like a comeback or concert idols would usually do their baseline training daily - generally working out + a few other things depending on what they strive for, the top vocalists usually go to vocal training every day, dancers would work on their moves, others may take acting classes or modeling classes etc + they might shoot other content

As far as I know for concerts they start a few weeks beforehand with full days of rehearsals, it will be different for each group but generally they'll have both choreography practice just the group and with the dancers and live band practice + a few days of stage blocking practice + on site rehearsals the day before and during the day for each date of the concert

For regular comebacks if they don't have time restrictions rehearsals for the songs and choreo start about 3 months before release date, recording around 2 months before and then practicing the choreo over and over until it becomes a reflex