Unless otherwise specified, in what direction and what duration of leg should be flown if assigned a hold?
Holding procedures are defined by each country for their own airspace. With that said, member countries of the International Civil Avation Organization (ICAO) generally follow the guidance of Part I, Section 6, Chapter 1 in ICAO Doc 8168, Aircraft Operations, Volume I, Flight Procedures:
Section 1.1.3 - The procedures described in this chapter are related to right turn holding patterns. For left turn holding patterns, the corresponding entry and holding procedures are symmetrical with respect to the inbound holding track.
While the document does not explicitly state that right-hand holding patterns are the default, it is implied.
Section 1.4.9 - The still air time for flying the outbound entry heading should not exceed: a) one minute if at or below 4 250 m (14 000 ft); or b) one and one-half minutes if above 4 250 m (14 000 ft). Where DME is available, the length of the outbound leg may be specified in terms of distance instead of time.
Member countries can deviate from these standards, but if they do, they are supposed to notify ICAO and publish these differences in their respective Aeronautical Information Publication.