It depends on the flavoring and how often it's exposed to air.
My Flavor West - some were 100% as strong as new after a year, some that were opened more often, seemed slightly weaker.
I've been dumping them at about a year since I discovered that.
I order at least 6 flavors at a time to spread the cost of shipping so the first 60ml bottle I make pays for that flavor.
In other words - I'd pay at least $10 for a 60ml bottle if I bought it instead of DIY and FW costs me $7-$8 per 4oz flavoring bottle