How do I change timing for NSTimer?

I have the following code:

timer = [[NSTimer scheduledTimerWithTimeInterval:0.50 target:self selector:@selector(onTimer) userInfo:nil repeats:YES] retain];    

-(void) onTimer 
{
}

After every 0.50 seconds the OnTimer method is called.

But now I want to increment the time-interval.

That means:

OnTimer calls after 0.55
OnTimer calls after 0.60
OnTimer calls after 0.65
OnTimer calls after 0.70
OnTimer calls after 0.75
& so on.

Is there any solution for this?? I have tried lot but its not working.