So, the question is - In order to give my client a reasonable time estimate, how long would it take to learn Java well enough, so that I can develop a complete business system? For those who switched to .NET, how long did it take them? I would guess the time estimated would be similar in both cases.'

Hilmar, an important factor is that the knowledge base for moving from VFP to .NET is larger than moving from VFP to Java.