Load the data into a table, then write the query inside out; with this kind of problem, nested queries are a boon.

1. Using Load Data Infile, load the text file into a table (indexless for speed) named tbl with columns for customerID, timestamp and amount, and index the table on (customerID, timestamp).

2. Write a query that tracks per-customer order by timestamp with user variables:

set @id='', @ord=1;
select 
  customerID, timestamp,
  if(customerID=@ID, @ord := @ord+1, @ord := 1) as custord,
  @ID := customerID
from tbl
order by customerID, timestamp;

3. Add an outer query that invokes #2 as a subquery to find every fifth customer row.

set @id='', @ord=1;
select customerID, custord
from (
  select 
    customerID, timestamp,
    if(customerID=@ID, @ord := @ord+1, @ord := 1) as custord,
    @ID := customerID
  from tbl
  order by customerID, timestamp
) x
where custord >= 5 and custord % 5 = 0

4. Add an outer query that counts result rows from #3 by customerID:

set @id='', @ord=1;
select customerID, count(*) as tokens
from (
  select customerID, custord
  from (
    select 
      customerID, timestamp,
      if(customerID=@ID, @ord := @ord+1, @ord := 1) as custord,
      @ID := customerID
    from tbl
    order by customerID, timestamp
  ) x
  where custord >= 5 and custord % 5 = 0
) y
group by customerID;