OP I think you are asking a really difficult question because there's no absolute time table per se (IMHO).
Everyone is different and everyone learns at a different rate.
Also, everyone is different and everyone approaches riding in a different manner (ie: some people like to take risks and some others are paranoid extra-careful freaks).
I dread to bring myself as an example here because the few people that know me on the board, know how sickengly fast I went through three bikes in the span of 6 months.
I started with a Ninja 250 just like you. I made the bad mistake of replacing it with a Versys 650 (NOT A 600!!!!!) after only 3 weeks. My reasons were valid but the bike was not. I made the mistake of buying a bike without test riding it ... or maybe I would have bought it nonetheless and found out the hard way that it wasn't for me.
After a few months of riding I stumbled in a great deal and landed on a Speed Triple.
That's a liter bike and it's got a massive amount of power.
However, I need to tell you about me, so we can put this into context:
I'm 40 years old. I'm an ex pilot. I'm extremely paranoid about screwing up (meaning: I'm a perfectionist by nature in everything I do). I literally printed out a checklist (similar to a pre-flight checklist I used for my aircraft back in the day) that I use to check everything in the bike and for my gear, prior to firing up the engine.
I go through the checklist every time I'm about to start the bike.
I have a different checklist for warm days, winter days, bad weather days.
I changed my checklist(s) when I changed bikes.
I studied the manual from the first page to the last and memorized all control panel settings that are pertinent and important while away from home.
I drive religiously at the speed limit, unless I have unlimited visiblity, no traffic, on the highway, in which case I allow myself to "punch it" and hit 80+ mph.
If the speed limit on the ramp says "20 mph" you will see me riding at 19 mph. I am not kidding.
To me riding is an exercise in perfection. Taking a turn fast is not the point. Taking the turn at exactly the right entry point and hitting the exact apex point is ... well, the point (pun).
Also as every decent pilot out there I religiously believe in recurrent training.
I will go back in spring to take the BRC with my bike this time (BRC-2 I think they call it).
Plus I will go to Poughkeepsie, NY to take the "total control" class.
Am I safe with a 135 hp bike? Yeah ... I think I am. Not because I'm an "ace" but because I have faith in my sense of self preservation.
Bad things can happen to anyone ... so if a truck pulls out of a blind alley at night, with its lights off at the same time a deer is crossing in front of me and I die because I get hit by the deer and run over by the truck even if I was doing 20 mph ... well, I guess my number was up (knock on wood ... I hope it isn't).
But I guess that my mantra is "I love riding ... I don't like dying" ... so if you ever were to ride with me, you'd get bored after half a mile, while I'd enjoy every second of my grandma-paced ride.
I am not saying that you need to be like me to be safe but in my experience a powerful and fast machine (be that a bike or a plane) can put you way over your head, way quickly. Unless you have the mental preparation and good muscle memory (that can only be developed over time and/or through training) to face emergencies or the unexpected, you might find one day that you go grab a fistful of brake in your supersport and find yourself flipped over, heading face first, into the SUV that just pulled into your lane ... good night and good luck. And all of that because you got startled and you didn't practice emergency braking and slow speed maneuvers with your supersport for 4 hours a weekend for a whole month the way I am doing with the Speed Triple I just got (again, because I'm paranoid).
A 600 can be a lot of bike, with a lot of brake power and engine power and unless you are ready for it, it can betray you and it might not forgive you if you make a mistake.
That's the whole point.
Sorry for the very long rant/post ... as you can see I gave a lot of thought to the subject.
unless you are ready for it ... and you may or may not be the best judge of that ...
PS: I MISS MY Ninjette!!! When people tell you to keep the Ninjette and have fun with it for a year, they are not kidding. If I could go back I'd probably keep the ninjette for a whole year. the Ninjettes are so much fun, so easy, so "sprinty" so low-key and low maintenance ... I regret selling it and in fact I am entertaining buying one just to keep it around for some weekend fun around town ... as a second bike. But I fell for the lure of more power and larger bikes ... oh well.
