Hello! My son will be 1 in a couple of weeks and so far he has had the same problem a few times now!
The first time, it lasted pretty much 3 weeks to the day, and started to slow down a day or so earlier then they just went back to normal over night! The second time was pretty similar but probably not as long, maybe 1-2 weeks! Both of these times he had multiple dr appointments and stool samples sent off. Always came back absolutely fine.
He has had a couple more of these bouts since, but since being on solid food they have been easier to deal with and not lasting as long, and thankfully making less of a mess which meant less frequent outfit changes. Us and his Dr have noticed a common theme when he is teething badly, this is when it happens, and we also have to get timodine cream on prescription for when he goes through a bout like this as it makes his nappy rash so so sore due to them being so acidic.
Luckily for me, my son never turns down a bottle of milk, so continued to act completely himself and stay hydrated during these bouts, so us and the Dr haven’t been worried about it for that reason, and also because his check ups and stool samples have all always been fine too!
However, and not to scare monger, my friends baby recently started with very similar symptoms to my son, but they very rapidly, within probably 10 hours, developed further symptoms due to a bug, which led to dehydration and went rapidly down hill during that time and has had to have some time in hospital on an IV.
If your baby develops other symptoms or doesn’t act like themselves, definitely take a trip to a&e or give 111 a call for an out of hours appointment if the symptoms are only mild. Mother’s gut always, always, knows best!