I'm not sure there is a universal answer to this.
It's not about the duration of "Free", it's about the emergence of a recognizable business model, and also about the general complexity of the product and how frequently I would use it and come to rely on it.
Another issue is that we all know that nothing is "free", and as the saying goes "if you're not paying for it, YOU are the product". I don't like the idea of business documents with my signature somehow ending up in an ad-based business model.
B2B sales models and tactics are (IME) very different than B2C. On the plus side, B2B is more likely to be receptive to a paid-for product, and the down side it can be somewhat more difficult to get the viral exposure you get from a B2C product since not as many people are likely to be interested in it, or able to decide for themselves if they can use your product.
A freemium model works good here. Have pricing set upfront and clearly defined. Let me test the product with no risk for some period of time and then convert to a paying customer (or not).