They have a minimum start level but they scale up.
every mission is dependant on the author. it could take 5 mins or it could take 3 hours (both extreme cases). i would say most people aim for the length of a standard episode, so 30-60 mins.
there will some minor xp from killing mobs, some dropped loot, and a minor reward at the end. there is a cryptic wrapper missions that lets you complete 3 foundry missions and get some proper xp and either a scaled item or some dilithium.
you can team with friends, but most people will tend to make them solo as its hard to test team missions unles you can rope friends into a lot of testing, but theoretically you can and should have few problems.
there is nothing to worry about. have a go. i have made a few that have gone over well. if you want an action one try 'the worst of all worlds', or if you want something with a bit of story and a bit more dialogue then try either 'lineage' or my new one 'dark ice' (or all 3 with the wrapper). I think they represent a good showing of what the foundry is all about.
just be kind when reviewing as foundry authors have limited tools to work with. we cant make missions exactly like cryptic's and sometimes we have to cut corners to make something work, so just keep it in mind. But i can saw that some of the foundry missions are amazing to play.
there is a cryptic wrapper missions that lets you complete 3 foundry missions and get some proper xp and either a scaled item or some dilithium.
Here's how you do it:
Pick up the wrapper mission Captain Revo speaks of. It is called "Investigate Officer Reports" on both the Federation and Klingon sides. You can remotely hail the contact from anywhere.
Having picked up the wrapper mission, now click on the tab for Foundry missions (I think it's called "Community Content" or some such). Choose missions that appeal to you. If you don't perform a search, then the most highly rated missions will appear at the top of the list. There are several fellow players who have made fine missions well worth playing, and you can see the authors' names before choosing the missions you want to do. Captain Revo is one such mission author; I can personally vouch for his missions being well done and enjoyable. Also, our own Grand Nagus maintains a list of recommended Foundry missions, and IMO you can't go wrong choosing items from his list.
Pick three missions that sound interesting and off you go! Once you finish all three, hail your Contact (the same one who gave you the wrapper mission). She will award you a nice chunk of XP and 1440 dilithium ore.
Again, you can do this on both the Federation and Klingon side, and there are different missions to do Klinkside. You might enjoy doing the same wrapper with a KDF character as well.
Edited to add: the wrapper mission is a Daily, so you can do it every day. IIRC, the timer resets every 20 hours.