calc.py

#!/usr/bin/env python                                                                                                                                                                  

rate=0.07 # some interest rate 1==100% .07==7%                                                                                                                                         
frequency=1/12.0 # yearly frequency                                                                                                                                                    
deposit_amount=200.0 # dollars                                                                                                                                                         
target=1000000 # dollars                                                                                                                                                               
current=100000 # dollars                                                                                                                                                               

def calcPeriod(rate,frequency,deposit_amount,target, current=0):
    """                                                                                                                                                                                
    Returns # of periods until target amount is acheived                                                                                                                               
    """
    periods = 0
    adj_rate = rate * frequency
    while target > current:
        periods += 1
        # apply interest                                                                                                                                                               
        current*=(1+adj_rate)
        # apply deposit                                                                                                                                                                
        current+=deposit_amount
    return periods

periods = calcPeriod(rate,frequency, deposit_amount, target, current)
print "Your net worth will exceed %.2f after %d periods or %f years" % (target, periods, periods*frequency)