Hello,
I researched this topic when my dad was diagnosed with a bladder tumor.
His tumor's size was first diagnosed to be 3cm (huge!)
The doctor said it was growing for about 2 years.
The average tumor growth is 1.4mm (millimeters) per month.
Dad's tumor didn't go into muscle, so it was great news for us. It was growing like a mushroom and could be easily scraped off.
Dad first complained about frequent urination in late December, went to see GP in January, then went to Uro in late Jan, then multiple more visits to CT, Ultrasound, Cystoscopy and in late Feb had a successful TURBT. (And a few days ago in mid April had 2nd TURBT and doctor said "it looked really good")