Really depends on the model of SSD. Some of them are more than capable of running near capacity without issues and some of them start to show inconsistent file write times if the drive is too full and you write too much data to them. I would say keep on putting stuff on there until you notice and actual difference in performance and back it off a little bit.
My M4 shows a noticable decrease in performance once I have less than 200GB free. At 150GB performance is actually terrible. Small writes can take 10's of seconds, the OS will lock up for periods due to the SSD locking up and its a bit of a mess. Its actually really disappointing to me that it happens with so much free space clearly still there, its just not acceptable really to be buying an SSD and finding you need to leave 40% of it free to get reasonable performance out of it. But this is an old SSD and now well known for these sorts of problems, a new generation one shouldn't show symptoms even remotely that severe nor with so much free space.