I would make a row number column and then use integer division to label the time periods.
library(dplyr)
DF <- data.frame(Name = paste0("A", 1:250), Value = rnorm(250))
head(DF)
#> Name Value
#> 1 A1 -0.08143561
#> 2 A2 -0.88270571
#> 3 A3 -1.29096226
#> 4 A4 0.56884325
#> 5 A5 -1.13031989
#> 6 A6 0.10579915
DF <- DF %>% mutate(Row = row_number() - 1, time_period = Row %/% 50 + 1) %>% select(-Row)
head(DF)
#> Name Value time_period
#> 1 A1 -0.08143561 1
#> 2 A2 -0.88270571 1
#> 3 A3 -1.29096226 1
#> 4 A4 0.56884325 1
#> 5 A5 -1.13031989 1
#> 6 A6 0.10579915 1
DF[48:52, ]
#> Name Value time_period
#> 48 A48 0.3865449 1
#> 49 A49 0.6577830 1
#> 50 A50 0.9418441 1
#> 51 A51 0.2948318 2
#> 52 A52 0.5437474 2
tail(DF)
#> Name Value time_period
#> 245 A245 -1.642056220 5
#> 246 A246 0.185185495 5
#> 247 A247 0.142535434 5
#> 248 A248 0.169160255 5
#> 249 A249 0.736191847 5
#> 250 A250 -0.005219281 5
Created on 2019-12-02 by the reprex package (v0.2.1)