[plain OM]

The coolant conditioner can be added at any time. Not having the conditioner doesn't mean the cooling system will fail due to its absence. (It's not like forgetting to refill the crankcase after draining the oil.)

Like many cooling system treatments, it's purpose (among several, depending on the formulation) is to seal tiny leaks that can develop. The conditioner doesn't prevent faults from developing; it only deals with them as they arise.

Some conditioners also contain lubricants that helps preserve the water pump seals, and some contain solutions to neutralize rust or deposit-forming chemicals that can develop in the coolant. But most good quality coolants (aka anti-freeze) have similar characteristics, and while any of these can become depleted over time, a couple of months is not going to matter.

The worst case might be if there was a tiny leak when you changed the coolant that the conditioner subsequently sealed, so there could have been a bit of coolant loss in the interim.

 

[drumboy]

I faced the same issue. While doing the timing belt job, I had my coolant flushed and refilled with non Subaru 50/50 coolant. Two weeks later, I talked to a service advisor at a local Subaru dealership who recommended only using OEM Subaru coolant. He also recommended the Subaru coolant conditioner. I ended up buying both and repeated the flush and fill job. He said that's the only exception to the "buy Subaru brand fluids" rule. According to him, Subaru coolant and conditioner together can help extend the life of the original head gasket.

 

[idosubaru]

Yes, just simply put it in later, you did the right thing.