If you have a big project, how do you know when it is time to be version 1.0?
Take any big project for an example: If it does what is it made for and it has not got many bugs, then it should be fine, but this depends on what you think and what other people post about it..
The traditional way is to make a list of the requirements. Tick the list of requirements, one by one, as each of them are satisfactorily solved. Once all the important requirements are satisfactorily solved, and the product has a satisfactorily low amount of open bugs and is considered stable enough for the intended primary users of the system, then you might consider blessing it as 1.0. There is no hard and fast rule, but that should give some guidance.
If following proper software engineering practices you would have (among others) a requirements document. Once the requirements of that document are met then you have version 1.0. After that you support it with newer versions (to fix bugs, add features, etc) as needed. There is much more to the whole process, but those are the basics.