Colonoscopy prep advice

I have a colonoscopy and I am taking 8 tabs senna and then a small sachet of citramag as prep. However it states that because I have a 7am appointment I need to start to take the meds at 1.30pm the day before. The issue is I can't get out of work until 3.30, wouldn't be till at the earliest 4 that I get home. I'm thinking to take half the senna tablets at 2ish then the rest at 3? I can't remember from the last time how long it took for them to kick in but does anyone know if I'll get away with this without disaster? My employer will get really fed up if I leave early because I'm taking two days off afterwards as I always feel terrible and have had so much other time off for appointments and sick days! I also have an hour drive (no pitstops for toilets :() to get to the hospital so I don't want to still be going after about 6am!

TLDR: how quickly does senna+citramag prep take to kick in? Need to start taking at 1.30pm but cant leave work or get home till 4! 7.30am next day procedure so need enough time for it to work as it's a 1h drive to hospital!