If you keep your mini-dv tapes out of direct sun light, and keep them in a climate controlled area, they should last a long time. I've seen estimates of 10 to 20 years for mini-dv. DVD life is still an unknown. Some claims state that good quality dvd's should last 20 - 50 years. I would keep them on mini-dv, and if they are important, every 5 - 10 years, copy them to your HD, then copy back to another (new) mini-DV tape. Best option, but not the cheapest, is to save in lossless format to a HD, then store the HD in a safe place. Also make a copy of the HD. But that can get spendy. You can store them on good quality DVD's, and recopy them every 5 years as well. Only problem there, is if you want to edit and reauthor the dvd, you will have some minor quality loss with each conversion.
Personally, I save my master mini-dv tapes, I save the lossless capture on an external HD, and then I have a couple of copies in MPEG2 format on DVD. I keep my external HD in my gun safe. I plan on recopying the actual dvd's (using dvdshrink or dvdfabdecrypter), every 5 years, or upon starting to see signs of the video degrading when it is being played.