Jeff Allison wrote:How long will an open bottle or port or madeira last?
It varies, and it varies a lot. A very old bottle of vintage Port might last mere minutes before it starts to decline (something I experienced with a bottle of 1873 Schofield Vintage Port) while a Madeira can last for years. I have a bottle of RWC New York Malmsey sitting on my small wine fridge that has been open for over a year and it's still going strong. Ruby Ports and Tawny Ports are also completely different... in general rubies last a few days while tawnies last a couple of weeks.
Jeff Allison wrote:Do LBV's and Vintage ports last the same amount of time?
More or less. At the very least they should show similar deterioration profiles since they're reasonably similar Ports. LBVs tend to last longer than Vintage Ports because they spent more time in barrel before being obttled, but the real variation comes about due to the age of the Port which leads us to...
Jeff Allison wrote:What about a younger bottle vs an older bottle?
As a general rule, a younger bottle of Port will last longer than an older bottle of Port. This is not as true of Madeira which is designed to be nearly bullet-proof in just about any circumstance you can think of. The quality of the Port will affect its longevity once opened; higher quality Ports tend to live longer than lower quality Ports.
In some cases even fairly old Ports need a significant amount of air time just to reach their full potential. A 1970 Taylor Vintage Port, for example, needs between 5 and 8 hours in a decanter to really shine. A 1980 Dow could probably use a good 12 hours. Either of those will then remain good for at least a couple of days - at the very least they'd still be great the next night.
Right now I have a Quinta do Portal 20 Year Old Tawny Port that has been open for a few days and it's still improving. I don't think it's going to reach its peak for a week... possibly 10 days. The first glass I poured wasn't ready, so I simply covered it and checked it the next night. Still wasn't ready, so I left it another day. 2 days after I poured it the glass was finally ready. The bottle, of course, had been re-corked on the kitchen counter and so hasn't had the same air exposure as that first taste, so the glass that I poured tonight isn't as nice... yet.
Jeff Allison wrote:Any general tips would be appreciated as I start to explore. Do I open with a group and drink over the course of a night, or do I open myself and taste over the course of a few days/weeks?
Try both! If you have a particular bottle in mind, post a question about decant time here and someone will surely have experience with it and offer help. When you open the bottle to decant it, pour some into a smaller bottle that you can seal air-tight and put that bottle immediately into the fridge. Decant the rest and enjoy with the group, then experiment the next night (or nights) with the small bottle that you reserved.
It takes a long, long time for most Ports to truly go bad. Unless you're opening something from the 1800s, it's going to be drinkable for weeks. When we talk about a Port going bad, what we're really saying is that it's going to go flat and be far less interesting. But you're not going to have to pour it out because it tastes like vinegar - I've enjoyed 3-week old LBVs before that still tasted fine!