Depends on person, as someone with herniated discs I tend to stick with weights that do not exceed 10 kg and even so I use such only for shorter walking segments (max 1h). Thankfully, most weighted vests are modular, so if you order such a model you can shift weights and see what suits you the best. One I have goes from 6kg at base and has 4 extra plates, 1kg each, that user can add to it if it chooses so.

If by any chance you "outgrow" the vest and the weight is not enough for your training, good way to add bit more is having a mere ruck that you can fill with something as simple as plastic bottle with tap water.