he naatha ramaNaSreStha
asmaakaM kaa gatiScaatra
bhaviSyati kalau yuge
"Ganga said: O protector, Supreme enjoyer, on your departure for the perfect abode, Goloka, thereafter what will be my situation in the age of Kali?"
varSaaNi tiSTha bhu-tale
paapaani paapino yaani
tubhyaM daasyaMti snaanataH
"The Supreme Personality of Godhead said: Remain on earth for five thousand years of Kali yuga. Sinners will deposit their sins in you by bathing."
bhaviSyaMti darSanaac ca
snaanaad eva hi jaahnavi
"By the sight, touch and bathing of one who worships Me by My mantra all those sins will be at once burnt to ashes, O Ganga."
puraaNaani bhavaMti hi
tatra gatvaa saavadhaanam
aabhiH saarddhaM ca SroSyasi
"There will be chanting of the name of Hari and reading of the [Bhagavata] Purana. Reaching such a place, attentively hear.
[note: In Puranic language, when "Purana" is used alone it refers to Bhagavata Purana. Otherwise it will specify Skanda Purana, Visnu Purana, etc.]
"Sinful reactions including the killing of a brahmana can be nullified be hearing the [Bhagavata] Purana and chanting of the names of Hari in the manner of devotees."
dahaMti paavako yathaa
"Just as dry grass is burnt by fire, by the embrace of Vaisnavas all sins are burnt."
tathaapi vaiSNavaa loke
pRthivyaaM yaani tiirthaani
puNyaanyapi ca jaahnavi
"O Ganga, the whole planet will become a pilgrimage site by the presence of Vaisnavas, even though it had been full of sinners and sins."
santi puteSu saMtatam
sadyaH putaa vasundharaa
"In the body of My devotees remains eternally [the purifier]. Mother Earth becomes pure by the dust of the feet of My devotees."
sadyaH putaani tiirthaani
sadyaH putaM jagat tathaa
"It will be the same in the case of pilgrimage sites and the whole world. Those intelligent worshipers of My mantra who partake of My remnants will purify everything."
maam eva nityaM dhyaayaMte
te mat praaNaadhikaaH priyaaH
tad upasparSa maatreNa
puto vaayuSca paavakaH
"Those, who everyday meditate only on Me, are more dear to Me than My life. The air and fire become pure simply by their indirect touch."
[Note: Sastra says that of all material elements, fire and air are always pure. Even though the air carries some impurities it always remains itself pure. This verse indicates that the Vaisnavas will purify even the pure elements of fire and air, therefore the purifying potency of the Vaisnavas referred to in this verse is not material but completely spiritual. I.e. the air and fire are _already_ materially pure, therefore the Vaisnavas purity is spiritual and not material.]
madbhaktaaH saMti bhu-tale
madbhakteSu gateSu ca
"For 10,000 years of Kali such devotees of Mine will be present on earth. After the departure of My devotees there will be only one varna [outcaste]."
"Devoid of My devotees, the earth will be shackled by Kali. Saying this, Krsna departed."
The above is supported in 4.90.32-33:
haris tiSThati medinI
devAnAM pratimA pUjyA
ZAstrANi ca purANakam
"(Sri Krsna said:) Lord Hari will stay on this earth for the first ten-thousand years of Kali-yuga. For that time the deities of the demigods will be worshiped and the Puranas and scriptures will also be present."
tad-ardham api tIrthAni
gangAdinI su niZcitam
tad-ardham grAma-devAZ ca
vedAZ ca viduSAm api
"For half that period the Ganga and other holy places will be present. For half that period the village temples and the Vedas studied by learned brahmanas will be present."
3102 BCE - Kaliyuga begins
+5000 - Ganga on earth (about 1900 CE, 414 Gaurabda)
+5000 - devotees on earth (about 6900 CE, 5414 Gaurabda)
(= 10 000 - Gaura era)
Interestingly, Ma Ganga is still present on earth so these dates may be subject to change, unless there is another explanation. One could be that BVT is focused on Rathantara kalpa.
Srila Prabhupada seemed to suggest a 10 000 years long era from his time, i.e. until about 12000 CE. For this I haven't seen any sastra pramana so far.