The rules for handling multiple IFR arrivals vary based on the type of radar services available at an airport and whether or not it is controlled. At class B and C airports, radar service is available to the surface and the control tower will either be worked by a dedicated controller or by the overlying approach controller. In those cases, aircraft may be spaced as tightly as 2.5 miles on approach to a runway and several aircraft may be cleared for an approach to that airport simultaneously. Each controller the pilot communicates with will be able to provide radar separation. At class D, E, or G airports, continuous radar service is not available because either the tower (at class D airports) is not able to provide radar separation or the pilots will be issued a change to the advisory frequency (at class E or G airports). In these latter cases, separation can be more challenging.
Normally at class D/E/G airports we use a “one in/one out” rule. This means that only one aircraft may be cleared for an approach or cleared to depart that airport under IFR. There are a couple of ways to work around this rule. The easiest is to have one pilot maintain visual separation from the other. With that you may have two aircraft arriving or departing IFR (or one arriving and one departing). Also, a tower controller may provide visual separation between IFR aircraft at his discretion.
For a busy class D airport, you might also use timed approaches. Timed approaches are used extensively in the real world to permit a steady stream of IFR arrivals, even in poor weather. These aren’t typically used on vatsim (but for an event involving a class D airport, their use should be considered). Timed approaches are covered in 7110.65 6-7; they require an operating tower, certain weather minimums, and that an instrument approach be utilized (visual approaches may not be used with timed approaches, unless other approved separation is used). 7110.65 6-7 talks at great length about using time between arrivals to ensure separation in a non-radar environment (hence the origin of the term “timed approaches”); in the real world there are places where aircraft hold outside radar coverage and thus ATC still uses timing for separating aircraft on the approach. On VATSIM, we always operate in a radar environment and therefore can disregard all mention of timing and just use the minimum distance between aircraft (5 or 6 miles) over the FAF/OM. Between similar performing aircraft, the 5 miles would be adequate; if a fast corporate jet followed a Cessna 172, that distance would need to be increased substantially (just as it would in a radar environment). Keep in mind that it’s possible both aircraft could end up flying the missed approach procedure as well. The spacing over the FAF/OM should consider that possibility at least to the point where diverging courses could be established in the radar environment. Timed approaches may only be used at controlled airports as the tower ensures separation between arrivals and coordination of missed approaches and IFR cancellation.