That's good you have started dream journaling and RC's, like Puffin said try the nose plug one

Also, with the mantras, its not really want you say, but how you feel when you say it and how much you believe it. Here is my tip, try saying "am" instead of "will" when you say mantras, and you have to really believe it when you say it for it to work.

There is no timeframe, like Puffin said again (damnit she beat me xD) its completely different for everyone.

For some specific induction techniques and a brief overview, here is a helpful page I recommend you read
An Overview of all Lucid Induction Methods - Lucid Dreaming - Dream Views

it seems you have had an LD already congrats