It can be done in one session (given the best of scenarios) but sometimes, not always, it cannot, not always in a single process anyway, but what I recommend to you is if you find out you CANNOT get it done in one single day/session/sitting, don't fear, just wear a hat if you don't want to be seen with in-between coloured hair til your next appointment (if your hair is healthy you may be able to get it the following week if not too busy) and I recommend you ask your colourist to add Olaplex into the bleach if they have Olaplex *ask about it and what it does you'll be drawn to it as soon as they explain it haha!* and this may be able to help you get it done in 1 appointment! But it's up to the colourists judgement if they want to process you so far, but if you cannot get Olaplex I'd honestly not worry about the colour it is left, don't go to fast with your processes, the faster/quicker you try to achieve it the more damaging it will be towards your hair and scalp, and some damage can be irreparable and unsalvageable (hopefully not though, hopefully your colourist is sensible and advises you the correct way to make your goal a reality!)
If you do it in stages with your hairdresser *with his or her judgement* your hair will be very forgiving to you and I highly recommend you buy professional shampoo and conditioner if you don't already as this will truly and ultimately make your hair look the same way it does when you come out of a salon, drug-store shampoo and conditioner will only deteriorate the condition and quality of your hair
There's always a 'short cut' but it's not always the best way!!
