We have ration for document and development. but we dnt hv any ratio for development and maintenance. As per my knowledge we cant define any ratio for this.
And neither can anyone else. I have been developing for over 25 years and Narue is right -- if you want a rule-of-thumb, or guestimate, then 1:60 (one minute development to 1 hour maintenance) is a reasonable one. Where I work we wrote a program 5 years ago and have been maintaining it every since because the customer keeps changing requirements and updating the hardware the program runs on. Its ok for us because we made tons of money on it and continue to do so every time the customer adds new requirements of changes existing ones, but that beside the point.
Narue and Ancient Dragon are correct. There is not a simple ratio. No developer can tell you that x minutes development results in y minutes maintenance. It just doesn't work that way. You say that you have a ratio for documentation to development. I would be skeptical of that as well. Real world development just does not work based upon such clearly defined ratios and any project is going to vary greatly. Such ratios are really only useful to people wanting to sell books on design methodologies and are often worth less than the paper upon which they are printed.
if we spend this much time for maintenance then when will develope new product...
software companies hire more programmers and is one reason for such a high demand for programmers. And is a reason for compiler writers such as Microsoft to invest a lot of $$$ in R&D (Research and Development) of new and improved IDE and compilers. Finally it is a reason for the huge demand for 3d party libraries and toolboxes -- to reduce program maintenance time and costs.