Haven't done that particular route
but have sailed in area. You didn't say where in Borneo you planned to leave from. It makes a difference, since Borneo is huge.
Assuming you're planning to leave from the nearest large port, Pontianak, and heading direct for Pattaya, the distance is about 800nm. Figure in a Mason 33 about 100nm per day in good conditions, or 8 days sailing.
in the region is conditioned by two distinctive monsoon seasons: (1) the NE Monsoon which brings generally wet conditions (rainy season) and; (2) the SE/SW Monsoon which generally brings drier/dry conditions (the dry season).
If your route
is to be from the western tip of Borneo, your course to Pattaya will be a bit west of north, so the SE/SW Monsoon (late May to September) would be more favorable, with drier conditions and with winds generally abaft the beam.
Lots of small islands in between, so you'll want to plan your route carefully.
The Java Sea in that season can be fitful...lots of calms punctuated by thunderstorms. Also, you can't rule
out the presence of 'unfriendlies' in the area. Suggest you consult the NoonSight website and other online sources for more details. Also, the Sailing Directions for the area. These include descriptions of the ports
and islands along the way, weather
For example, the Sailing Directions for NW Borneo say this about the weather:
The Southeast Monsoon sets in over the S part of the strait in
April, blowing from the NE to SE. Calms and NW winds are
sometimes felt. This unsettled weather lasts until June, when
begins to blow with some regularity from SE,
occasionally shifting to the SW. At night during June, the wind
is mostly E to SE, but it also blows from the S and SW. These
winds will bring in a swell and, blowing in opposition to the
prevailing S currents, produce a short and troubled sea.
9.1 In September and October there is a decrease in wind and
sea, and changing to S and W in November, and in December
from SW to NW. These winds cannot be relied upon.
9.1 The Northwest Monsoon is at its height in January and blows
from the NW. Heavy squalls, rain, and heavy seas occur and
then begin to abate in February. Light breezes from NW to NE
and E occur in March.
9.1 December to March are considered the rainy months, but
there are showers in all other months. July to September can be
termed the dry season. Squalls and thunderstorms occur in
9.1 In the N part of the strait the force of the wind is less.
9.1 The monsoons from N and SSW are variable and depend on
the strength of the wind circulation in the Celebes Sea and the
Java Sea. The seasons are not well defined and plenty of rain
9.1 The SSW winds commence in May. These veer occasionally
to W and NW and are variable and unsteady. The monsoon is at
its height from June to September, and SSW winds prevail both
day and night.
9.1 In October, the monsoon decreases in force and is lowest in
November, the wind sometimes veering to the N and NE. In
December the general direction is NW, in January N, and in
February NNE winds blow, decreasing in April when calms... Unquote
Hope that helps a bit.