When did cavalry soldiers unsheathe swords?

The cavalry tactics could differ from century to century, yet when they got firearms, for sure, they had to learn quick swapping from firearm to a saber during the attack.

The Russian cavalry officer Nadezhda Durova (aka The Cavalry Maiden), who fought in Napoleonic wars, mentioned some details in her well-known memoirs. That is how she depicted the standard training:

надобно было первому перескочить ров, выстрелить из пистолета в соломенное чучело и тотчас рубить его саблею

The first one had to jump over the trench, shoot the pistol into a straw dummy and to chop it immediately with a saber

She doesn't say whether she unsheathed her saber only after shooting yet it seems that jumping over the trench with both hands full is too complicated. That is fast weapon change is a must.

Surprisingly later while talking about one of the battles she writes exactly what you are looking for:

Эскадрон наш ходил несколько раз в атаку, чем я была очень недовольна: у меня нет перчаток, и руки мои так окоченели от холодного ветра, что пальцы едва сгибаются; когда мы стоим на месте, я кладу саблю в ножны и прячу руки в рукава шинели: но, когда велят идти в атаку, надобно вынуть саблю и держать ее голою рукой на ветру и холоде.

Our squadron did several charges, which made me very displeased: I have no gloves and my hands grew numb because of cold wind, so my fingers could hardly bend; when we are staying in place I put my saber in sheath and hide my hands in the sleeves of my coat, but when they order to charge it is needed to put the saber out and to hold it with a bare hand under the wind and the cold.

I can't find the true reason for this - was it a kind of a psychological attack or something - but her words are pretty clear. Charging with sabers unsheathed is an authentic warfare method even in XIX century, even though the cavalrymen used the pistols for range shooting and could put out their sabers very quickly.

UPD.

For one of the latest examples of saber attacks, there is also a well-known "Saber fight of 13th Kuban cossack cavalry division" in WWII on 2nd August, 1942 near Kushevskaya, Soviet Union. One of the participants, Lt. Serdtsov, wrote about it:

Артиллерийский огонь стал стихать, в воздух взвились одна за другой три красных ракеты. Командир полка подаёт команду: “Поэскадронно! Развёрнутым фронтом! Для атаки! Шагом марш!”
Когда полки дивизии развернулись и приняли строгий боевой порядок, и стали проходить поле с неубранным ячменём и подошли к лётной площадке аэродрома, комдив полковник Миллеров, выхватив из ножен шашку, сделав над головой три круга, выбросил вперёд шашку и перешёл на аллюр-рысь. Заблестело море казачьих сабель. Противник открыл ураганный огонь из всех видов оружия. Казаки переводят своих лошадей в полный галоп. Раздаётся громовое “Ура!”. Когда лётное поле уже закончилось, фашисты перед лавиной казаков вздрогнули, выскакивая из лесопосадки, начали в панике убегать в сторону станицы по направлению элеватора и ж.д. станции. Вот здесь и началась “работа” — сечь.

For those who can't read Russian, Lt. Serdtsov actually says that they had put the sabers out while their horses still went trotting and charged much later on a full gallop. Later he also mentions his gun "attached to a pommel" which he used after his horse was shot dead. The Soviet premium lists also mention the actual use of sabers like "chopped upto six enemy soldiers and officers", "saved the officer by chopping off enemy's hand when the latter was aiming parabellum" etc.

UPD.

Here is The Drill Regulations of the Red Army Cavalry, 1938 which provides enough details on cavalry attack technics.

The chapter 1.6 "Attack" gives the full sequence of orders for "the normal attack":

Шашки к бою (Sabers to battle)
В атаку (Attack)
(Аллюр) (Pace)
МАРШ-МАРШ (MARCH-MARCH)

The cavalrymen do unsheathe the sabers on the very first order; the horses go full career on the last one.

There's also a "short" form for a sudden attack: В атаку, МАРШ-МАРШ (Attack, MARCH-MARCH). Again, the first part is actually an order to unsheathe the sabers, while the last one - to go full career.

The "normal" attacking distance (for "MARCH-MARCH" command) is thought to be about 200-300 meters.