IMO, it's an individual choice.
First, if you're a market timer, keep cash when the market is high, but if you aren't a market timer this doesn't apply. I'm not going to say you should be one or the other.
The comment about holding 30+ months cash during a downturn (and I know you didn't say that was you) doesn't make sense though. In a downturn you would be putting cash back into the market unless you thought it was still going lower. Most likely this is a factor of illogical thinking, that when the market is down you are afraid of it and won't buy low, instead waiting for it to get higher before getting back in. Not a strategy to replicate.
The other factor is how likely you are to need ready money. Do you foresee any big expenses coming? Kids who might need help?
A lot of people talk about emergency funds, but I don't think those have to be cash. Have enough for the more common emergencies if you have them, but if the unexpected happens you can always sell stocks when the market is high, or bonds when it is low. I have so few emergencies that I'd rather have my money working for me and take whatever hit, if any, to sell funds if I have to. Sounds like you're retired so you don't have to worry about replacing income due to job loss.
If it makes you feel better to have some cash in place of bonds, go ahead, it probably doesn't have that much impact.
I'm actually probably close to 15% cash myself right now, but a lot of it is tied up in CDs as an alternative to bonds. I don't even think of this as "cash" though, since it's not readily accessible without penalty.