Learn Finnish: Katselisin installaatiota niin kauan kuin kriitikko on täällä. - I would look at the installation as long as the critic is here.

Katselisin comes from the frequentative verb katsella (to watch, view, or look around), which implies an ongoing, continuous, or relaxed action. Katsoisin (from katsoa) would mean a simple 'I would look'. Because you are viewing an art installation over a period of time, the continuous katsella is the natural choice.

Why is installaatiota in the partitive case?

The verb katsella strictly requires its object to be in the partitive case. This is because the action of viewing something is inherently continuous and conceptually incomplete, unlike a sudden action that might take an accusative total object.

The current lesson is about joskin (although). How does niin kauan kuin differ, and could we add joskin here?

Niin kauan kuin means 'as long as' and defines the timeframe of the action. Joskin introduces a concession ('although' or 'albeit') and serves a totally different grammatical role. You could easily add a joskin clause to the end of this sentence to qualify it, for example: ...joskin vain kaukaa (...although only from afar).

Notice the tenses: katselisin (conditional) and on (present). Why isn't the second verb conditional too?