If your startup suffers the chicken and egg problem, you can easily do it part time, because it'll be next to impossible to gain users. So you can have it on the side, until you grow the userbase to be able to monetize it.
However if you are doing something involved with direct sales, every hour you spend on the startup = # of new sales = $$$$$
Comments
it depends on your startup type.
If your startup suffers the chicken and egg problem, you can easily do it part time, because it'll be next to impossible to gain users. So you can have it on the side, until you grow the userbase to be able to monetize it.
However if you are doing something involved with direct sales, every hour you spend on the startup = # of new sales = $$$$$