Is doing 10 times 10 pull-ups during ~1 hour 5-6 time a week too often? Is it good for keeping current muscle level, combined with the right diet? Is it necessary to add weight to grow muscles? What will decreasing frequency and increasing the amount per exercise change?
No, it's not harmful. You can workout 7 days a week, twice a day if you really feel like it. Just make sure you use good form and if you feel you're getting tired/fatigued really easily and struggling with the workout, take a day or two off to recover/relax. Listen to your body. Make sure you consume enough food and water. High intensity, regular exercise can take its toll on some people - some much quicker than others. Everyone is different.