I get divits in my tips, normally takes about a month. Normally i just put the thing on a grinding wheel and try and smooth it out. They don't work as well after but if you make sure and tin the tip after you grind it it'll last a bit longer.
The time it takes to fry a transistor depends on the particular part. You may want to check the data on the maxium temperature. Most transistors you have ample time to solder them before you harm them.
Solder tips are usually made of copper for good heat conduction and are coated with a layer of iron to protect the copper. When the iron coating eventually wears thru, the copper tends to erode rather quickly. I have this happen from time to time on my Weller here at home and also on my Pace MBT station at work. JB
as to question 1, I use a fairly high power iron (its gets extremely hot- too hot for electronics, but hey...) and have never had any problems, but if you are worried, you can get 'heat shunts' which are bassically reverse action tweesers, which you can clip on to the transistor legs to take away some of the heat. Or a pair of locking forceps works quite well
I did once melt a transistor, but thats another story...