Bullock's character actually spends very little time fully sealed in the Sokol suit; she generally has the helmet off or open. The longest time she spends sealed in the Sokol suit is while she is clearing the parachute lines holding the first capsule to the ISS. In the image below, it's clear that she's plugged into the capsule's LSU through the tether line:
In the "fire extinguisher" scene, where she uses the extinguisher to get from the capsule to the Chinese station, the scene is done as one continuous "take". It takes her approximately 3 minutes to get inside the station, which is tight but technically do-able even just holding her breath. The biggest problem with the scene is that IRL it takes several minutes to pressurize an airlock; she could easily have died of asphyxiation while waiting for it to cycle.
The Sokol suit is designed to be plugged into an external life-support unit. The only artistic license was not to bother showing her plugging and unplugging from the capsule's LSU, which could be confusing to audiences that are used to thinking of all spacesuits as fully self-contained units.
According to this Space Foundation.org article :
Designed in response to the suffocation death in 1971 of three
cosmonauts aboard the Soyuz 11 mission to the Soviet Space Station
Salyut, the Sokol space suit was first worn by Soviet cosmonauts in
1973. Unlike bulky EMU (spacewalk) suits, the sleek Sokol suit can be worn in the exceptionally tight quarters aboard the Soyuz spacecraft
where there is no room for cosmonauts to wear extensive life support
The suit, which weighs 22 lbs. (10 kg), consists of an inner pressure layer of rubberized polycaprolactam and an outer layer of
white nylon canvas. Boots are integrated with the suit, but gloves are
removable and attached by blue anodized aluminum wrist couplings. The
polycarbonate visor can open on hinges mounted near the ears and seals
with an anodized aluminum clavicle flange; the hood folds when the
visor is raised. The suit has four pockets and adjustment straps on
the arms, legs, chest and abdomen.
The Sokol has a suit pressure gauge on the left wrist and a mirror on an elastic wrist band that helps the wearer see things otherwise
outside his/her field of view. During re-entry, an altimeter on a
wrist strap may also be worn to measure cabin pressure and alert the
wearer when to brace for touchdown. A wristwatch is often worn as
well, with an elastic wrist band replacing the strap so it may fit
over the bulky suit glove.
Electrical cables are mounted on the right abdomen of the suit; separate hoses for air and oxygen are on the left. An electric blower
ventilates the suit with cabin air and, if the cabin pressure drops,
the air supply is automatically replaced with oxygen from pressurized
bottles. Air and oxygen exhaust through a pressure relief valve at the
center of the chest.
The suit is intended to be worn for up to 30 hours in a pressurized environment or two hours in a vacuum; it can also float
and has a neck dam that allows the visor to be raised in water without
the risk of flooding the suit.
The current version of the Sokol has had the mobility issues improved, and it is possible to manually adjust the relief valve (the blue thingy in the middle of the chest area) to compensate for the "ballooning" effect for short periods (<15 minutes) , although it's not recommended.