Without an ND filter, your options are limited. First, use the least light possible: set ISO to minimum and stop the lens down as far as practical. You can also shoot when light levels are lower, such as near sunset or after the sun drops lower in the sky, which allows longer shutter speeds.

Another workaround is to simulate a long exposure by taking many shorter exposures and averaging them in software. Stacking the images with a mean/average blend can create a smoother water look similar to a single long exposure, as long as the camera stays perfectly still.

As for the “needed” shutter time: there isn’t one fixed value. The right time depends on how fast the water is moving and how smooth you want it to look. Faster water needs less time; calmer water usually needs more. Without an ND filter in bright daylight, you may simply not be able to reach the necessary shutter speed in one shot.

So in practice: lower ISO, use a small aperture, shoot in dimmer light, or stack multiple frames. If you want the effect reliably in daylight, an ND filter is the proper tool.