countMonths <- function(day, month, cycle.days) {
  month.days <- c(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
  if (month < 1 | month > 12 | floor(month) != month) {
    cat("Invalid month value, must be an integer from 1 to 12")
  } else if (day < 1 | day > month.days[month]) {
    cat("Invalid day value, must be between 1 and month.days[month]")
  } else if (cycle.days < 0) {
    cat("Invalid cycle.days value, must be >= 0")
  } else {
    nmonths <- 1
    day.ct <- cycle.days - day
    while (day.ct > 0) {
      nmonths <- nmonths + 1
      month <- ifelse(month == 1, 12, month - 1) # sets to previous month
      day.ct  <- day.ct - month.days[month] # subtracts days of previous month
    }
    nmonths
  }
}

> head(cons2[-1],10)
   kwh cycle.days  read.date row.index year month day kwh.per.day
1  381         29 2010-09-02         1 2010     9   2   13.137931
2  280         32 2010-10-04         2 2010    10   4    8.750000
3  282         29 2010-11-02         3 2010    11   2    9.724138
4  330         34 2010-12-06         4 2010    12   6    9.705882
5  371         30 2011-01-05         5 2011     1   5   12.366667
6  405         30 2011-02-04         6 2011     2   4   13.500000
7  441         32 2011-03-08         7 2011     3   8   13.781250
8  290         29 2011-04-06         8 2011     4   6   10.000000
9  296         29 2011-05-05         9 2011     5   5   10.206897
10 378         32 2011-06-06        10 2011     6   6   11.812500

> dput(head(cons2[-1],10))
structure(list(kwh = c(381L, 280L, 282L, 330L, 371L, 405L, 441L,
290L, 296L, 378L), cycle.days = c(29L, 32L, 29L, 34L, 30L, 30L,
32L, 29L, 29L, 32L), read.date = structure(c(1283385600, 1286150400,
1288656000, 1291593600, 1294185600, 1296777600, 1299542400, 1302048000,
1304553600, 1307318400), class = c("POSIXct", "POSIXt"), tzone = "UTC"),
    row.index = 1:10, year = c(2010, 2010, 2010, 2010, 2011,
    2011, 2011, 2011, 2011, 2011), month = c(9, 10, 11, 12, 1,
    2, 3, 4, 5, 6), day = c(2L, 4L, 2L, 6L, 5L, 4L, 8L, 6L, 5L,
    6L), kwh.per.day = c(13.1379310344828, 8.75, 9.72413793103448,
    9.70588235294118, 12.3666666666667, 13.5, 13.78125, 10, 10.2068965517241,
    11.8125)), .Names = c("kwh", "cycle.days", "read.date", "row.index",
"year", "month", "day", "kwh.per.day"), row.names = c(NA, 10L
), class = "data.frame")

> cons2$tot.months <- countMonths(cons2$day, cons2$month, cons2$cycle.days)
Warning messages:
1: In if (month < 1 | month > 12 | floor(month) != month) { :
  the condition has length > 1 and only the first element will be used
2: In if (day < 1 | day > month.days[month]) { :
  the condition has length > 1 and only the first element will be used
3: In if (cycle.days < 0) { :
  the condition has length > 1 and only the first element will be used
4: In while (day.ct > 0) { :
  the condition has length > 1 and only the first element will be used
5: In while (day.ct > 0) { :
  the condition has length > 1 and only the first element will be used

cons2 <- ddply(cons2, .(account, year, month, day), transform,
               tot.months = countMonths(day, month, cycle.days)
)

mapply(countMonths,cons2$day,cons2$month,cons2$cycle.days)

cons2$read.date=as.Date(cons2$read.date)
monnb <- function(d){ lt <- as.POSIXlt(as.Date(d, origin="1900-01-01"));  lt$year*12 + lt$mon }
mondf <- function(d1, d2)  monnb(d2) - monnb(d1)
mondf(cons2$read.date-cons2$cycle.days,cons2$read.date) + 1

countMonths <- function(day, month, cycle.days) {
  month.days <- c(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
  stopifnot(month >=1 & month <= 12 & floor(month)==month & cycle.days >=0 & day >= 1 & day <= month.days[month])
  nmonths <- 1
  day.ct <- cycle.days - day
  while (day.ct > 0) {
    nmonths <- nmonths + 1
    month <- ifelse(month == 1, 12, month - 1) # sets to previous month
    day.ct  <- day.ct - month.days[month] # subtracts days of previous month
  }
  nmonths
}