You can keep a starter for 10-14 days IMO without having to re-start it.
A starter should be done in a 10:1 ratio . For example, a 1L starter you put 100grams DME in the flask and then fill to the 1L mark, shake to dissolve, boil 10 minutes, cool to 70 and pith yeast. You can then put it on a stir plate or shake frequently. Loosely cover the flask with a sanitized piece of foil.