library(plyr)
library(ggplot2)

t <- function(N) {
  ldf <- llply(seq_len(N), function(n) data.frame(n=factor(n)))
  ldfb <- rbind.fill(ldf)
  NULL
}

adply(
  data.frame(N=2000 * 2 ^ seq_len(4)),
  1,
  function(N) {
    system.time(t(N[1,]))
  }
) -> timing

timing

##       N user.self sys.self elapsed user.child sys.child
## 1  4000     1.655    0.006   1.670          0         0
## 2  8000     4.370    0.002   4.397          0         0
## 3 16000    14.370    0.002  14.452          0         0
## 4 32000    59.716    0.023  60.168          0         0

timing$kN <- timing$N / 1000
lm(user.self~kN+I(kN^2), timing)

## 
## Call:
## lm(formula = user.self ~ kN + I(kN^2), data = timing)
## 
## Coefficients:
## (Intercept)           kN      I(kN^2)  
##       1.641       -0.203        0.063

ggplot(timing) + geom_point(aes(x=kN, y=user.self))

sessionInfo()

## R version 3.0.2 (2013-09-25)
## Platform: x86_64-pc-linux-gnu (64-bit)
## 
## locale:
##  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
##  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
##  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
##  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
##  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
## [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] ggplot2_0.9.3.1.99 plyr_1.8.1         knitr_1.5.22      
## 
## loaded via a namespace (and not attached):
##  [1] colorspace_1.2-4   dichromat_2.0-0    digest_0.6.4      
##  [4] evaluate_0.5.2     formatR_0.10.4     grid_3.0.2        
##  [7] gtable_0.1.2       labeling_0.2       MASS_7.3-29       
## [10] munsell_0.4.2      proto_0.3-10       RColorBrewer_1.0-5
## [13] Rcpp_0.11.0        reshape2_1.2.2     scales_0.2.3.99   
## [16] stringr_0.6.2      tools_3.0.2