How long is a piece of string. It depends on many factors, not least of all is your skill level.
Personally I break down the spec in to tasks; eg. onboarding, user registration, login, reset password, change password, profile editor, avatar upload etc. Then I estimate hours for those tasks. Then I take my hourly rate an multiply the two together and add a bit for contingency.
Also, if you're doing this for a third party client, make sure you spec out exactly how many hours work you will do for spec changes, and how many hours support you include, and the ongoing support hourly rate. I've seen too many freelancers bitten because they didn't let the client know that support is not indefinite, and spec changes will cost extra.
Also note that you'll need some server-side code to handle the contact form for you. Bootstrap is only a front-end library.