Fred Wilson venture capitalist and Co-Founder Union Square Ventures
“[With a fast growing company], doubling employees year over year, adding users and customers  very rapid[ly] ,  don’t  raise too much money.  [Otherwise] [the company] will be sitting on cash  raised [at a lower valuation]  [which is] too dilutive to [founders] and angels.
[Wilson has] two basic rules of thumb [for the amount to raise in early stages, i.e., seed, Series A and B rounds]. First try to dilute in the 10-20% band whenever you raise money.” 10% is preferable. More may be necessary, “ but try  to keep  dilution below 20% each round. If you do two or three rounds [exceeding] 20% each round, you’ll end up with too little [equity].
Second, raise 12-18 months of cash each time you raise money. Less than a year is too little.  Longer than 18 months means you may [have cash when the company had at a lower valuation].
 When [a] company gets above 100 employees and valued at north of $50mm, things change. You may need  more cash  for working capital  and [the company] may not be increasing value [as rapidly as] when [it was] smaller.” A raise of 24+ months cash may then be appropriate. Fred Wilson, How Much Money To Raise, Jul 3 2011; http://www.avc.com/a_vc/2011/07/how-much-money-to-raise.html